My existing tiles can't be re-grouted? What are my other options?

[MTiler]

Ive used Grout re-colourants, theyre very good and very easy to use, but on very old, stained grout it can end up a bit patchy, deffo do a good clean first.
The tiling ideally needs to be very good and spot on to have a contrasting grout colour otherwise it'll show up every dodgy bit.

 

[mean_in_green]

The contrast of black will make the grout lines jump out so they would need to be nice and consistently sized to look right. If their width is too variable you might want to leave them white.

 

[AMtek]

use a stanley knife and a big pack of new blades then re grout

 

[jSPENCEINC]

Lowes sells a electric grout removal tool, i usually pass on regrouting jobs if the price isnt right, but recently took on a 300sq/ft master bath and had the same issue with manuel grout removal tool, after one tile chip i picked up what lowes had to offer and it worked like a charm