I tend not to see that many people playing with the GBA Castlevania games enough to know this offhand. While I have played the games at this point I can't really remember much about them so I am going with the basic thought process.
As a general rule.
Is there a bonus mode/new game+ featuring them? Is there a section of the game where they have theoretically complete control (not just a walking only section, or given it was 2002 maybe a stealth section)?
If there are such things then chances are the sprite has all the relevant animations for all the weapons (as opposed to a NPC that might just walk and never use a weapon).
If no then you likely get to create all the "missing" animations. Just looked at a sprite sheet and if so then... while not the worst it is still going to be a fair bit of work.
Anyway if yes there is a fully fledged mode then you can approach it how you will. Obvious first step would be see if there is a character selected value somewhere in the game. These things are easier to find when there is a simple transform button or in game item but not impossible without it. Even if it is a bonus mode the dev might well have just got lazy (very possible with the portable castlevanias if my experiences and those of others are anything to go by) and coded something that is effectively a character swap. It could be anywhere or with anything in RAM.
Lazy option is overwrite or repoint the sprites, and presumably change the text in the script where necessary. This assumes characters with the same abilities as well as obviously just changing graphics will not do a lot for abilities.