Can I alter the peice? like put a diffirent figure on it, aka mage knight.
without changing base at all

There is not a thing in the world stopping you from altering a piece in any way you see fit.

Some judges, however, won't let you play an altered piece in a tournament, though, and MOST judges won't if the piece's base has been changed or altered in any way.

Me? I'd let you do it if the base wasn't altered, and if the piece in question wasn't TOO misleading... a Savage Hulk base with a Sasquatch figure, for example, would be fine. A Savage Hulk base with a Wasp or Shadowcat figure would be totally unacceptable...