When can they apply again before current visa expires?

I don't see any issue with applying for the visa early. If the advice is to apply 3 months before travelling, they can't do that if they wait until the old visas expire. The visa application asks for your travel dates.

The only issue is that they will be wasting some of the "investment" in their old visa - but if they weren't going to travel between November and March then it's wasted anyway.

Some countries have a fast-track visa process for the UK, but it can get rather expensive, so if you have time, just go for the standard process.

Be careful to provide ALL the documents requested - bank statements, proof of employment, details of property, other family members etc. They need to be able to prove (a) that they can afford to come /stay in the UK and (b) that they will go home afterwards. If you provide a support letter, you need to also provide all the same documents as they are asked for.

Thanks Filka! Yes, they know about the process as they did it several times now but this is the first time that they are going to apply before their visa expires, hence my question.
Thanks again for your help

Yes when you apply again it just overrides the old one. It's also possible to extend a visa if you haven't stayed the full 6 months on the given visit, but that is excessively expensive so the new visa would be cheaper.
As a related point, if you can show they have a reason to visit repeatedly in the long term it might be worth applying for a 10 year visa. This doesn't really save money on the visa, but it saves the hassle of applying again.

Hi Johnd2 and thanks a lot for taking the time to respond. You're right, applying for a UK visa is a real hassle and applying once every ten years is way better than say every two years.
Cheers

Hi again!

Mum (Dad didn't apply) has been issued a 2 year visa as requested starting from January 2020 but her old visa, expiring in March 2020 has NOT been cancelled. Now she wants to visit the UK in December. Is her old visa still valid? The most reasonable answer is yes, her old visa is still valid, as she was expecting her new visa to start in November and her old visa to be cancelled. But we're still confused! Please help me understand!

It seems logical that the old one is still valid, but it isn't worth the risk of arriving at the airport to be told you can't travel.

Can the visa centre which issued the new visa not answer this?

Thanks filka for your input.