If you have a Pro version we suggest you use Tag editor to rename tags. In tag editor, you can go to Individual tags tab and rename Project Y to Project Z and it will be renamed in both instances.  

The following works in all versions:

Let's say that at one point 'Project Y' was renamed to 'Project Z' and you would like to rename it everywhere you've used it in the past.

1. Go to some day where that tag was used.
2. Right click the tag on tag timeline and choose All instances of ... -> Rename.
3. Type the new name and press OK.

Let's say you have these two tags:
'Client X, Project Y, Deadline discussion'
'Client X, Project Y, Design'

If you want to rename Project Y to Project Z, you need to rename both of them via All instances of... So you need to do this for every tag combination Project Y is a part of.