Ríonfénnid: one unit from Sáthach's host may change its Acuity to become Trained, if it is Levy; Drilled, if it is Trained; Veteran, if it is Drilled; and Elite, if it is Veteran; all for 0 cost per warrior.
Obviously, putting Sáthach in charge of a unit of Gairlom would mean that they would be drilled, rather than trained. If I'd already paid for the upgrade from trained to drilled, however, could Ríonfénnid turn my drilled Gairlom into veteran Gairmorlom?
I can see arguments for (it's basically the same unit) and against (it's a unit with a different entry in the muster). I'm sure there must be other units this would apply to, as well, this is just the one I'm thinking about.