should I perpetuate bigint? I have a base table used everywhere called Customers.  Of course, its unlikely that table will ever grow past int size, but it is keyed on a bigint nonetheless.

I don't believe that I can change the type of this identity column to int.  But what is your advice for building stored procedures, table type variables etc that refer or carry data for this column?  Should I perpetuate bigint in these routines?  Will they run leaner if I use int, leaving it to the engine to run implicit conversions?
It is always a recommended practice to keep the parameters to be the same data type. This is to prevent potential implicit conversion issues with predicates which can lead to the query optimizer not being able to utilize statistics, or indexes, which leads to poorer performance, and can lead to serious CPU and storage performance issues.