How to get a micro SIM and keep the same number and provider?

I've finally took the decision to upgrade to a smartphone but the phone I'm looking at (Nokia Lumia 435) requires a micro SIM, at the moment, my current phone has a standard SIM and I don't want to change providers or number. I don't really like the idea of cutting it, will I get a new SIM and have to change the number myself or will they deal with it in the shop I'm getting my phone from.

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