Replacing Grout or Sealant
The first thing to say about removing grout would be that it will take you a long time... But if you have spare time, it's definitely something you should do!
Remember this, to start, you need to make sure you properly get rid of all the silicone before you attempt to apply the sealant, this will ensure that the applied layer will adhere correctly and stay waterproof.
To attempt this task, you can either use manual tools, which will be affordable and fairly easy to use. Otherwise, you can opt for an electrical grout remover which will save a monster amount of time but needs a bit more precision and expertise, as well as a larger sum of money.