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.
- Go to some day where that tag was used.
- Right click the tag on tag timeline and choose All instances of ... -> Rename.
- 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.