Hi,

It depends on the code inside the custom module, if you take some apps from a version and installed in higher version sometimes it will work fine without any issue. Suppose if the custom module just add a new field some model and view, most likely it will work across version.

So in your case, just take the custom module from the previous version and install it in the latest and see whether it get installed or not. If installed successfully check whether it works as expected or not. If not get installed and raise some errors, you can fix errors one by one and go ahead till everything is okay.

You can have a look at this explaining the same: How To Migrate Odoo Modules To Another Version

Thanks

Hi Haris: 

Since the upgrade is between major releases, there is a very high likelihood that you may need to make changes to your custom code to make it work correctly. In an case, you should always do rigorous regression testing to make sure that everything works as intended before doing this in production.