There's no reason you can't still go retro even if you have a new style A2 lower. Lots of pictures I've seen posted here are of completely retro builds on A2 lowers. There are only two minor differences in the look of the lower so it is hard to tell unless you are looking really close.
That said, the A1 and A2 handguards are interchangeable. Either the slip ring or delta ring will work and the A2 handguards are made to work with the A1 triangular endcap. There are three projections on the front of the A2 handguard that allow it to work as either the top or bottom, unlike the A1 being either left or right.
This modification made logistics much simpler for the supply sergeant as you didn't have to specify left or right, the A2 handguard will work in either position, top or bottom.