This is a common issue encountered when importing contacts from an external file into any number of different starter solutions.
Here's a thread on how to fix it: Help importing Excel data into a Filemaker Starter solution
Thanks so much for your help! Now, my only problem is that Replace Field Contents isn't available as a choice from my Records menu (it's listed but it's showing as grey, which means I'm not able to choose it).
It will not be grey once you put the cursor in a field. Put it in the field you want to update. (And this cannot be a field of type calculation either.)
Make sure you have a back up copy of your file before you try this just in case you make a mistake.