The first index for anything in the IT world is "0", not "1". Most certainly any programmer in this world expects the index "0" to be the first one.
An implementation must add 1 to each index, and the FIX engine then decrements it (will not kill the CPU but it is at least some work that could be avoided).

My solution would be to deprecate the methods and have new replacing methods (remove the deprecated methods and rename the new ones in a later version).