I believe the upgrade is random, so you might get normal cloth 20 from the 15, or it might end up going exceptional.
Other than that, the BOD type won't change, so you probably can't turn cloth to leather, or ring to chain or plate.
The 3 things you can bribe are BOD-count, quality and material - so cloth BODs and weapons probably won't upgrade much, or upgrade in unwanted ways, since material change isn't an option for them.