Using New Universal Login with customized JS

Previously I was using Classic Universal Login with customized JS for the login/password reset pages. However, I want to be able to use Email as a backup MFA factor, which means I need to switch to the New Universal Login.

I don’t want to lose my customized login/password reset pages, so can I switch from Classic → New Universal Login and still use my custom JS login/password pages??

It seems like things still work when I switch to New Universal Login and my custom JS pages still load the same on my site, but i do see this notification

“One or more of your Universal Login pages are currently using custom HTML which override the New Universal Login experience.”

Is this something I should NOT be doing? Will things randomly stop working as updates come to the New Universal Login page?

Thanks in advanced!