My upper appears to be USGI A1 configuration, the lower is a Double star. I really like it and enjoy shooting it.
What kinds of upgrades are available? I don't mean new barrels and stocks, rails, glass and such.
Specifically I'm referring to what retro-fit parts are there that have been 'beefed' up to reduce stoppages, prolong service life  etc such as springs, extractors, pins bolt parts etc. The little things that can be replaced without changing the aesthetics of the rifle but will improve it's reliability and function.

TIA

the A1 is just like any other modern AR15/M16 in terms of parts interchangeability.  

as far as reliability upgrades, you can rebuild the bolt with the ejector spring/buffer upgrade and new gas rings.  or replace it if it has high mileage and makes you feel better.  maybe new buffer spring if you suspect high mileage on the ones you have now.

not a lot else that i would proactively do for maintenance without getting into the upgrade realm which then basically allows you change every single part on it.

I have to agree with what Lionel has said. As mild as the A1 is to shoot, there is not a lot of cause or need to do any upgrades. If you have worn parts, by all means replace them, but other than that, keep it as is. That is my opinion.

Thanks guys. Mine appears to be in the middle of its lifespan so no immediate need to replace anything. When I do I want to put in the best replacement parts available. :)

Nodak Spud for a Retro lower receiver would be the first thing some of the folks here would suggest.  Not everybody thinks that's absolutely necessary but there's no denying they do really good work.

Way down the road, when you finally shoot your barrel out, there are options for replacement tubes.

One of the Retro-heads here did a pretty dang good job of putting together this database of barrel manufacturers/suppliers:

http://www.ar15.com/forums/t_3_123/676721_Retro_Barrel_Source_Spreadsheet.html