really? i have always been told that you have to use the correct base size for the model based on the current rules, otherwise it creates issues for other players trying to measure range to you.
'tis a hassle, as 2 armies i have have had this happen to them.. my 'nids (biovores and more the guard apparently) and my Tau broadsides.
at least they aren't squads I have a lot of models in.
lol this would be mind breaking if it happened in fantasy. "Beastmen gors are now on 20mm bases!"
From my understanding of it as long as you have the old models, you are not forced to replace the bases, though the rulings on that seem to be mixed by area.
lol definitely gamed that that guy/girl.