Please contact iFastNet to get this resolved for you.
In theory you should not worry about the free account being suspended, but sometimes the system can go wrong and mistakenly pause a domain that have already been moved to premium.
Just ask iFastNet to change the DNS for you again should do the job.
Is that also the domain name you’re actively in using? If so, then you’ve been paying for a premium account you’re not using. You’ll probably want to contact iFastNet to get your domain and website migrated to your premium account.
Which domain are you using on premium hosting? If it’s a custom domain, please check that it’s using iFastNet’s nameservers, not free hosting nameservers. Any free subdomains from iFastNet are definitely unaffected by this.
If so, I’m pretty sure that that domain will keep working when the free hosting account is deleted, although you could ask iFastNet to be sure.
Still, I see that the free subdomains redirects to a different website now (brainz.uk), which is using the registrar’s nameservers while pointing to a premium hosting IP. So that website will definitely keep working no matter what happens on free hosting.