There may be a conversion guide for pre-1D&D subclasses, but I doubt it will work terribly well. If they are in fact standardizing the levels where you get subclass features (which makes perfect sense and opens up a lot of cool design space), then any mapping of older subclasses to the new system is going to be kludgy at best -- some features will be over-leveled, others under-leveled, and what do you do when the new class and the old class have different numbers of subclass feature slots?
On top of that, the old subclasses were designed to work with base class features that may change or be removed.
The best way to use 5E subclasses in a 1D&D game will most likely be to use it as part of the 5E class it was designed for. From what we've seen so far, 1D&D and 5E characters should be able to play at the same table with few or no issues.