Also to clarify some of the confusion regarding compatiblity:
To support a given network 2G, 3G, 4G etc both the phone
and the SIM have to be compatible. Your current SIM (presuming it's very old) will likely only be 2G/3G compatible, and presumably that's true of your current phone too. All current phones capable of 4G or 5G is very likely to be back compatible wtih 2G/3G too (best to double check the specs though - for which I recommend this site
https://www.gsmarena.com/ ). Hence my advice above to change phone and use the existing SIM until you get told it'll stop working then get a 4G (more likely it'll be 5G now) SIM.
There is no account to transfer. Your number is registered against both your current SIM card and your phone's ID. The latter the network doesn't care about, you can use any SIM in any phone that isn't network locked (very few are these days). So there is literally nothing to "transfer" or "migrate", so ignore what the Nokia website is saying. When it comes time to change to a new 4G/5G SIM, you just go through the steps your network provider tells you to follow. Very likely will be either a) we'll send out a new SIM for you to use, or b) they may get you to commit to a new 18-24 month SIM contract. Either way your current number SHOULD be retained (it's part of their licence conditions that they cannot impose number changes on customers).
Forgot to mention. The one problem you may run into is that SIM cards have changed physical formats over the years, they've gotten smaller. It's possible (likely) any new phone now will not support your current SIM cards physical format. Old SIMs were larger and new phones tend to take the newer smaller SIM formats.
There are three different SIM card sizes and this guide will talk you through the differences between them.
www.4g.co.uk
Eg, these three modern 4G compatible Nokia's all use the smallest SIM format:
www.gsmarena.com
www.gsmarena.com
www.gsmarena.com