I think officially you'll have to reload those ESXi hosts to enable a new vSAN trial period. There's not going to be any "official" way to reset your trial, even if you destroy the vSAN config and re-enable it.
No, there is no supported way to reset this license and a new one can't be applied - adding capacity is actually the only thing that won't work after it expires (but obviously activities such as replacing disks, enabling or disabling features such as dedupe/encryption also won't work). Unfortunately you are going to have to re-image those nodes and get a new trial-license - there *potentially* are methods of removing nodes from inventory spoofing that they are new hostd and adding them back to vCenter and applying new trial-licenses but probably more work a than re-install. If you work closely with or have good relations with vendors that might be able to supply NFC licenses that would be better, one gets full NFC VMware suite licenses from vExpert, but anyone can get them cheap as part of VMUG Advantage membership IIRC.
ok will probably leave this host as a compute only (at least for now) as looks like its going to require a lot of time and all test VM's are on vSAN.
Wished there was a quicker solution for tests environment but at the same time I can understand why there probably isn't/or is not easy.