Google Mobile First Index algorithm, what will happen to my site

My site (example.com) has pages where Desktop and Mobile are DIFFERENT pagessitting in the same site. Mobile visitors to the site are redirected to the Mobile version which is not indexed (with NOINDEX, NOFOLLOW meta directives).

Mobile page is ranking anyway because we think Google is using the Desktop page for its ranking algorithm. It will not be like this when Mobile First Index gets enabled.

The question is: what will likely happen to my mobile rankings if I don't do anything?

I'm planning to get the mobile page indexed and use switchboard tags to interlink both versions but I don't want to do anything drastic as the pages are already ranking well. Creating a responsive page to replace both is not a good solution for me as both versions are very different.

Thanks

Why are you using old methods for mobile sites? Invest in having someone optimise your entire site for both.