I am currently in the process of regrouting a bathtub…
Don’t make the mistake I did.
Everything was going well, with scraping out the old grout (as blairrl520 wrote, you don’t need to scrape it all out, just a little bit). But then, when I applied the new grout, I (apparently) didn’t apply it correctly… I am now–2 months later–still scraping off the excess grouting material from my tile, trying very hard (and only sometimes succeeding) not to scratch it.
My knuckles are bloody, and I can’t imagine how or when I am going to finish this job… What a mess!