Will becoming a subscriber automatically add Dynamic Lighting to past created games using supported modules?

If I become a subscriber with access to Dynamic Lighting, will games I've created in the past using official modules that support Dynamic Lighting (LMoP, for example) be updated to include the Dynamic Lighting barriers? What if I add a new, supported module to an existing game that was created previously? Will the light and barriers already be drawn on the map as it would be if I started a new game with a new module, or will I have to manually draw them at this point?

Yes. The dynamic lighting lines and light sources are already in the modules, they just don't do anything if you don't have dynamic lighting access.

Also to add, if your subscription stops and then restarts all dynamic lighting (even self-made ones) are accessible again. i was slightly worried I'd lose those but thankfully it was all great

Can't tell what you're asking, since all modules are "supported" by Dynamic Lighting.  If you buy an official module and add it to your game, it will have Dynamic Lighting included only if the author specifically placed Dynamic Lighting onto the maps themselves. Creators typically sell their packs in two versions, with and without the lighting (the lighted versions normally costing more). The majority of marketplace items do not include Dynamic Lighting, but there is a growing number of rereleases of old packs that now include Dynamic Lighting for an additional cost. "Official" modules (e.g. from WotC) are almost always setup with Dynamic Lighting before release (however, these are occasionally subject to error).

The Foegetter said: Can't tell what you're asking, since all modules are "supported" by Dynamic Lighting.  They were asking if they purchased a module during a time in which they were a free subscriber, would all the pre-set DL features of that module become manifest should they later subscribe, or would they have to re-add the module. As the previous answers have attested, yes, any pre-set DL features would become immediately available in an installed module or addon once a subscription of Plus or Pro begins. They are always there, just inaccessible at the free level.