Hey Oscar - I have two thoughts.

(1) If you have Explore, you could write a query using the Ticket Updates dataset. Use the "changes" attributes to select the status and assignee fields and new values of solved and the assignee (you will need the assignee ID, not the name - you can grab it from the user profile). Then add another field/filter for the ticket current assignee NOT being the user you are talking about. This should show you that the user solved the ticket but isn't the current assignee. Add Ticket ID to the query and you have the list!

(2) If Explore doesn't work for you as an option, I might make a view for tickets updated in a time range (from when you made the change), the status is solved, and the assignee is not the assignee you changed. If you sort by update date, you should see all the ones changed at the same time you made this change (assuming there have been no other updates to these tickets). If you feel these are right, you can "check" the tickets and bulk update the assignee on them all.