I have sold more firearms than I could recount right now, upwards of 40-45 I would guess. Only really regret 1 (Colt 5" Metropolitan .38 Spl) so I have no issue with you selling your .308 if you want a AR15. Selling guns to buy other guns if pretty common practice it seems.
I would just say that with the glut of AR15s on the market right now your might want to look as buying one and keeping the .308, there are some really great deals on ARs right now for not much money. And of course you could just buy it piece by piece to budget it out if you want.