FIRST, the easiest thing is just to delete the activity and create it again. Then make the necessary associations and your done... THIS is what I would recommend....
WARNING use this approach with extreme caution and ONLY after you have backed up your .eap file...
YOU CAN screw up the data in the file and leave your project unusable. I would NOT recommend you doing this if your not familiar with querying or looking at tables using MS Access, or your not comfortable with poking EA in it's innards.
But if you are STILL interested and you have MS Access installed, (or another application that allows you to look/modify the data in an Access database,) then you COULD do the following... NOT that I would... but, (Get the idea that I don't want to be responsible yet?)
Change the EAP file's extension to "mdb", then double-click it, Access will launch and “might” ask if you want to convert the database. Make sure you ANSWER No!
Then, simply look down the list of tables until you see the table called “t_object” and open it up... Filter on the column “Object Type” column for "Activity", then find the activity you want to fix by by name. Look in that row’s e column titled "NType" and the value displayed most likely is 8, just change it to 18.
Now get the heck out and don’t change anything else…Exit Access, rename the file to have an extension again of .eap, double click it and see if that did the trick...
AND remember I did NOT tell you to try this…

Bioform