Hi Stuart,
Perhaps you have already tried this (and I suspect it does not fully correspond to your needs), but just bringing awareness to the use of global change (Views>Activity List>Task List>[F10]) to change multiple activity parameters at once. I don't think it has the flexibility to identify and change a specific part of an activity name across different names, so this will depend on how your activities are named. Pairing this with a filter is potentially a solution you could use, again, depending on how your activity list is set up.
You will likely find that fixing this via Excel is the preferred method, especially as importing directly from Excel is simple.
Regards,
Tierney