Changing the grout on porcelain tiles

[Biscuitsforbribes]

Hello,

We've moved into a property with lovely white porcelain floor tiles which have been grouted by the previous owners with white grout.

We've recently tiled the kitchen and have used a grey grout, and would love to do the floor to match. Is this possible to do without damaging the tiles? They've long been discontinued and we weren't left any spares, so if we do damage one we're screwed. No budget to retile completely and don't want to as the current ones are really lovely.

I've asked my dad whos a builder, but he grimaced and said he thinks it wouldn't work but I don't know if that's true or if he just thinks he's in the firing line for doing it

Thanks!