If you have pact of the blade can you change your summoned weapon mid-fight by unsummoning it and resummoning it?

Yes, assuming you haven't turned a magic item into your pact weapon. Keep in mind it takes an action to summon your weapon, so you're losing a turn doing this.

Ok, so would this be like drawing a weapon or is it more like casting a spell?

Summoning your pact weapon is a special action, not one of the standard actions listed in the Combat chapter of the Basic Rules/Player's Handbook.

If something is a spell, the rules for it will say so.