If you want to simply replace this type you can redeclare _TKey in Unit1 as Integer. So whnever you will be refering to _TKey in Unit1 or other unit that uses Unit it will be of Integer type but refering to _TKey from Unit2 will return you variable of String type.
Note these would be two completly diferent types not interchangable with each other who would only share same name. They can't even exist in same code scope.

But if you want to have one common type to which you could acces either as Integer or as String then I'm afraid you have to use generics (Type Helpers: http://wiki.freepascal.org/Helper_types). I'm afraid these are not available in Delphi 7.

Maybe you could make use of classes and their getter/setter methods to do necessary type conversion when you need it. That depends on how your other code is designed.
If you care to share what exactly you wanna achive we might help you out with a different aproach.