Multiple enumerations of IEnumerable
possible multiple enumerations of IENuemerable
Should I change it to a List or leave it the way it is?

This depends on the situation. If there is an EF query behind the IEnumerable and you have never called .ToList() on it (not even once), then every time you enumerate it, the query will be executed against the database. If this is your case then you need to call .ToList() so your query can be executed and the results can be stored in a list on the application side. Whenever you want to get a fresh copy from the database, then you can run your query again or run a query which gets you the delta.
If you are doing everything in memory and there are no database calls, then if items are added to the underlying list, you will not get them. Consider this snippet:
IEnumerable&lt;int&gt; list = new List&lt;int&gt; { 1, 2 };
list.Count(); // returns 2

list.ToList().Add(3);
list.ToList().Add(4);

list.Count(); // it still returns 2