Deleting preferences might solve it, as that procedure often cures issues where iMovie is behaving in an unexpected way. So try that and see what happens. It is a harmless procedure.
If deleting preferences doesn't cure it, the redownload iMovie procedure would get you the highest iMovie version that your operating system will run. So, the version would be compatible with El Capitan. El Capitan is a 32 bit OS and you would be updating/redownloading an iMovie app that is compatible with 32 bit. That shouldn't cause you to lose the functionality of any of your 32 bit apps, versus what would happen if you upgraded your OS to Catalina or newer, that are 64 bit.
Since you are running El Capitan, the redownload procedure is slightly different. Everything would be the same except that for El Capitan and older, click on the Purchases tab at the top of the app store screen and then sign in. Click on Purchases again to reveal your purchases. You should be able to redownload from there.
As a workaround, put your cursor on any audio clip that may be below and attached to your photo clip that you want to be replaced and, while pressing down, pull the audio clip down to a lower audio well until you see the little attachment coupling pull off of the photo. (If the audio clip is attached to a different photo, you can skip that step.) The point is to have a free photo of the audio clip so that you can delete the photo without also deleting the audio clip. Now that the photo is free and independent of the audio clip, drag your replacement clip to the timeline and place it right after the clip that you want to replace. It should squeeze right into the timeline, pushing the other clips aside. Then, select the clip that you want to replace and press Delete on your keyboard. That will delete it and it will be replaced by the replacement clip that you inserted into the timeline. It will slide over into the space vacated by the deleted clip. Adjust the duration of the replacement clip to match that of the deleted clip. That should accomplish what you want without disrupting the rest of the timeline.
-- Rich