I have a cell c number (with nashua mobile) that I want to keep... can I walk into any cell shop and ask them to retain my number on a different network (new contract)?
...or is it more complicated than that?
Yes you can keep it. MNP was introduced for exactly this reason. However, just make sure that your contract with Nashua Mobile has expired before porting from Cell C to another network.
If you port before your contract expires, you will still be liable to pay the remaining subscription fees on your Nashua Mobile contract. Also, your contract with Nashua will be unusable even though you're still paying for it, since it won't be linked to any tel. number. So if you have bundled minutes, SMS, data, etc, you will lose it.
My personal recommendation is that if your contract has expired / is about to expire, then don't renew it or sign a new contract, but rather port to prepaid (either Cell C, or another operator), and wait a couple of months for the new packages and new phones to be released. With interconnect rates coming down, and the
imminent launch of Telkom Mobile, I wouldn't want to be caught up in a 24 month contract, paying ridiculous fees for the next 2 years because I signed on the dotted line, while everyone else is enjoying lower call rates.. But that's just my opinion...