Hello GGOVEK hope you are doing fine.
I'm not sure I understand your issue. Can you please elaborate more?
Is there any way to go back from vCLM to VUM? Short answer is No. You can re install a previous version.
If you want to roll back the ESXi version you can use the following proceess
Reverting to a previous version of ESXi 6.x and 7.0 (1033604) (vmware.com)
Because I have NSX-T and now I can't upgrade ESXi hosts
This shouldn't be an issue.
What version of ESXi, NSX and vCenter are you using?
Are you using VCF?
What is the error you are recieving?