In fairness its probably better you stay out of it completely, it can be a mountain my Partner, Thai Government Teacher, sole carer for elderly parents, house 6 x farms a 10 year old son, north of 1 million bhat in the bank should really have applied without my support, sponsor letter proof of my funds etc strong reasons to return to Thailand but refused as they did not believe the relationship. Advised to re submit sent in around 200 photos relationship work on house vacations screen shots of what's app conversations etc and it was granted. I fear that if your niece has no provable way to show she will return to Thailand then you may well struggle. Whilst I don't necessarily say it will be the case UK immigration may possibly read between lines and draw inappropriate wrong conclusions. If your niece could apply for a tourist visa just in her name without your sponsorship I feel she stands a far better chance.
A quick check of your main page lists you as being in Ireland if SAGA is available there go for that its a lot better than any product in Thailand. You can get annual cover that allows for trips of 90 days I went for a Policy to the value of 10 million pounds aged 64 1 pre existing condition and the premium was a little under £300
I believe I have read online that it is only possible to do the conversion in country at I think and I could be wrong here Bangkok number 1. There is however one specific office in Thailand ( Bangkok) that will convert your visa subject to meeting the requirements for the visa you wish to change to.