WotLK Transmog leather to plate - need info

Hello guys! So i got Mok'Nathal Beast-Mask and I really want to transmog my Relentless plate helm to it but game won't let me. I think this shouldn't be the way it is as I'm allowed to wear leather armor anyways. Now, I remember I used to see a lot of people back in cata realm transmoging swords into axes and such. I think they used a macro or something. Could you tell me whether this will work on Icecrown and how it needs to be done exactly. Also, is this banable?

Thank you!

plate to plate leather to leather etc. bypassing that would likely count as bug abuse/ an exploit and result in a ban.

It's not like you're using a 3d party program or anything ... and people used to do it a lot. That's why I'm actually asking. I don't think this could get you banned back then.