Hmmm... I think that despite maintaining their armies during the Star League era, each House Lord knew very well that attacking the SLDF with his army would be .....at least ....dangerous but probably more akin to suicide. So I think its possible that the House Lords out of pure spite maintained their armies but didn't modernize them much causing the House armies to field mostly sub-average equipment(at least compared to the SLDF).
Pure spite? Not exactly...
Furthermore why invest loads of money into your own army if the SLDF is obliged to come to your rescue?
Hidden Wars anyone?
Outside of perhaps the first few decades of the Star League's existence (say until Leonard Kurita died) the Great Houses have always viewed the Star League as a means to an end (especially during its later years.) They've always invested, complained and clambered for advanced weapons technology (basically anything on the HRAD's "No No List") during the League's existence.
Keep in mind that the Great Houses (including the Hegemony) aren't altruistic by a long shot. They're greedy, selfish and xeno-phobic. They hate one another with a self loathing born out of cultural differences and centuries of warfare.
Remember, the Star League represents just a small break from the traditional cycle and only replaces the old open confrontations with new confrontations occurring just below the surface of outright open-warfare for only a century and a half.
(the last 25 years of the Star League's existence could be different, though, but to modernize an entire House army within a few decades would, IMHO, be challenging to say the least. Both financially and technically.)
Not that much of a challenge really. Look at how quickly the Great Houses retooled and produced advanced equipment during and after the Clan Invasion...sure the tech renaissance began in the late 3030s and took two decades (and a war) to fully mature, but keep in mind that before the First Succession War the Houses are at the height of their industrial and scientific power. The construction and implementation of advanced technologies isn't a physical problem...not even a little one.
The only real reason(s) why the Great Houses wouldn't saturate their armies with advanced tech is simple: Fear.
During the Golden Age, the Star League could impose economic sanctions and other forms of monetary punishment (prior to the double up edict) on any House attempting to challenge the SLDF's dominance. Keep in mind I said Star League, not Terran Hegemony. Individually the Great Houses could also make things difficult - spare parts not arriving on time, Terran equipment costing more, etc.
Not to mention the likely arms race that would ensue as a result of any one House attempting such a build up and those corresponding consequences...such as intense economic disruption. Combined, intertwined economies anyone?...
During the Golden Years, fiscal greed outweighed the possible benefits of upsetting the status quo. As long as the Houses could achieve their ends through relatively peaceful means, outright flaunting their military strength or engaging in a military buildup was counter productive. But once faith in the Star League, aka their ability to swindle and gain through political means, waxed...all bets are off.
Still, even until the late 2750s there's still the fear of the SLDF. And with the Terran Hegemony still around and at its economic and scientific peak it would be the likely winner in any arms race. As a result the Houses satisfied their growing disdain and individual need with military growth - as in doubling their personal armies.
So yes I can see the House armies being not so well equipped prior to the Succession War.
There might have been some showcase units in each of the House units(say Sword of Light for example) but the bulk of the House armies would field rather outdated equipment.
I totally agree with this. Showcase units, despite being a part of an army's TO&E are also viewed as points of national pride and sometimes fall outside what observers would call a "normal" military unit. Upgrading your flagship regiments is NOT the same as upgrading your run-of-the-mill line regiment.
Still, there's a problem that hasn't been addressed by TPTB and likely never will. One, that would answer this question definitively.
We know how easy it is to upgrade and build Star League-era tech and to replenish the ranks thanks to the Clan Invasion. Something like what, 2-3 decades tops?
So how "upgraded" do you think the Great Houses could have become from 2766-2779 and then from 2780-until the First Succession War?
If we apply the same rational used by TPTB for the Clan Invasion, the answer is: quite a bit.
But, even if the Houses only upgraded incrementally during the Amaris Coup (from 2766-2779 since there was no way of knowing the Star League would end) they'd still have the better part of two decades to prep for the First Succession War after it was over.
I mean, as early as 2781-82 the House Lords would have seen the writing on the wall and known that the League was over...and as I mentioned before, they were at the height of their production and scientific means. Plus, we have Terran expatriates defecting from a devastated Hegemony (even the Republic will suffer a "brain-drain.")
So I'm thinking the Houses can upgrade quite a bit. The only thing really slowing them down would be the economic upheaval caused by the Amaris Coup and the subsequent breakdown in relations between the various Great Houses. However, an easy counter would be to point out the previous decades prior to the Coup when the Great Houses were effectively raping the periphery economically and stocking their individual coffers to the brim.
All in all, I'd lean more towards seeing the Great Houses fairly well outfitted with Star League tech by the start of the First Succession War.