How to replace a role by another one in a specific process ?

Here is a partial solution. It saves you redrawing and layouting the connections 30 times, but still you have to do the replacement 30 times:

Click on the role name "Project Manager" of the role occurrence like you wanted to change its name. Type a few letters of the new role's name and then either select the new role from the instant search list, or hit the Tab key, when only one result is left. This won't change the name of the role "Project Manager" then, but instead exchange the role connected to the symbol with the new role. The symbol size stays the same and the connection does not move.

After your changes don't forget to do a reorganization of your database (if that isn't done automatically at regular intervals in your installation) in order to get rid of the Project Manager's former connections on the definition level.

Thanks for your reply. This is exactly what I would like to avoid. I have curently assessed roles and I have more than 1000 objects to change.

Any automated way will be appreciated.

I see 1000 is a different number than 30. In that case it would be worth writing a little script (or have it written for you). That should be quite simple to do, if you have someone with some scripting experience.