What you should expect
What should you expect trading your M1a for an AR10.
1, Expect tighter groups, the AR 10 from the factory is more accurate. To get an M1a to group as well you must spend a lot of money.
2, Expect quicker follow up shots, the AR 10 straight line stock does not rise as much in recoil as the M1a. (one of the reasons the M14 was a failure as an automatic battle rifle.)
3, If you reload, expect much better case life. Typical case life for an M1a is three reloadings. Beyond that you is on yore own. AR 10, has a much tighter chamber, thus works the brass much less which equal longer brass life.
4, Expect a far wider range of accessories, goodies, doodads, bells, whistles and more for the AR.
5, Expect to be able to safely use all factory .308 ammo. M1a rifles are well known to function best with powders of medium burn rates ( H or IMR 4895 is a known good start) M1a rifles are also known to damage themselves if the wrong powder or load is used. (read slow burning powders with heavy bullets) Bent operating rods are a common result of the slow powders.
6, Expect that your AR10 will be US made. no chinese made major parts.
7, Expect that it will be easy to mount optics of an enormously wide range on your AR 10. Not even remotely so with the M1a.\
8, Expect that you will be using a more modern design firearm, benefiting from many years of advancing firearms design & metallurgy.
9, Expect that there will be many that will disagree with some or all of the above. In many cases, tradition trumps innovation.
10, Expect that you will regret trading / selling you M1a at some time in the future. You really need both.
Roger