MTN issues - cancelling contract

Been having a few issues with MTN lately where my 3G/HSDPA connection isn't very usable. Certain things will work like whatsapp and other messengers but Facebook and Google won't. I'm assuming there's something wrong with my sim card and trying to get hold of MTN is ridiculous.

What I want to do is go with Afrihost mobile.

First question: Can I get an Afrihost sim elsewhere besides getting it delivered from Afrihost? The MTN stores don't sell them I'm assuming?

Second question: If not, can I grab a new MTN prepaid sim, cancel and downgrade my contract to pay as you go keeping my number. Then do a number port with Afrihost using my old number to the new MTN(Afrihost) sim card?

Will this work even though I am already on the MTN network with my current number?

Thanks