Actually, for the plains units, I think you should exchange them for exp, both N and R. Make sure you exchange always 10 girls of the same kingdom, as it grants a HUGE bonus (Exchanging a unit usually nets 1,25 exp per gold (not including exp bonus which grants +1000 exp without cost), but selling 10 lvl 1N/R units grants 9200 exp for 2400 golds (so ratio three times better).

You will have enough plain SR for upgrading them to UR fast enough (I have already 3 of them and the other 5 are coming soon).

Upgrading plain R to SR is a huge waste of exp.