The tile contractor didn't do a good job while setting the tiles in my
kitchen; in some locations the grout doesn't fill the joints properly -
it's too low. (I know it should be slightly lower than the tiles, but
in these cases it's too much).

I would like to regrout the floor to fix the problem. Is it OK to do
this over the existing grout (was set couple of months ago)? Is it
going to stick to it? These are polished porcelain tiles, and grout is
less than 1/8" thick.

Thanks,
Matt