Glad you found this useful.
You'll have no problem converting an X/Y recording to Mid/Side and back again. You can do that with any L/R stereo signal actually, it's just that doing so with a signal which is free of phase-differences like that produced by a coincident pair avoids potential comb filtering issues when converting back to L/R using a different ratio. That's not to say one shouldn't try the M/S manipulations on recordings which do have phase variations, such as those made with near-spaced or spaced techniques. Only that one should listen carefully to make sure the M/S correction isn't introducing more problems than it's solving.
To expand a bit more on this, that kind of L/R > M/S
(do something) > L/R conversion is useful for other things too. The
do something part doesn't have to be a change of M/S ratio. It allows one to manipulate a two channel stereo signal in terms of "the stuff in the middle verses the stuff at the outside of the stereo image" instead of in terms of "Left versus Right". So one can for instance, EQ the the "center" to bring out some needed vocal clarity, differently than the ambience and instrumentation which images more widely. Which is why many EQ plugins now feature a built in Mid/Side mode which does the conversion (and back) for you. Or do things like spectral edit out ambient hall noises from the Side channel which are heard ambiently in both channels but not in the center. Or whatever.
For some reason - I had the idea that M/S would be more forgiving.
Getting back to this ^, usually there is a certain range of ratios which just work and sound best. Trying to adjust too far either way causes things to not work so well in stereo (unless one wants to extract just the Mono sum or the Side component for some purpose). So Mid/Side recording (and post manipulation) is best thought of as providing a stereo optimization tweak rather than offering a huge range of potential adjustment. Remember that the virtual microphone angle and the virtual pickup pattern remain inextricably linked. Changing the M/S ratio changes both these things together. One cannot change the virtual angle while keeping the virtual pickup pattern unaffected too, and vice-versa.
The ultimate extension of Mid/Side which does offer infinite, independent adjustability is ambisonics, which is in many ways the ultimate coincident microphone setup. In it's basic form, ambisonics extends Mid/Side by recording and manipulating 3 or 4 recorded channels instead of just 2 and allows one to point as many virtual microphones as one wishes in any direction one wishes, choosing the pickup pattern for each independently, as well as selecting any 1st order pickup pattern along the continuum from omni through figure 8. Recording 3 channels allows for that adjustability around the entire horizontal plane. Recording 4 channels allows that adjustability for the entire 3-dimensional spherical space around the microphone array, meaning you can adjust the vertical angle of the virtual mics as well. The caveat is that all virtual microphones remain coincident, one cannot introduce near-spaced or spaced phase-difference stereo without using additional non-coincidently located mics.
Conceptually, and sometimes in practice, a basic 3-channel ambisonic (horizontal plane) microphone setup consists of a pair of bidirectional microphones crossed at right angles and an omni, all arranged coincidently. It's like the Blumlein technique I described earlier combined with a coincident omni. You may recall that all microphone pickup patterns can be thought of as the combination of omni and bidirectional components (pressure and pressure-differential components) combined in various ratios. Ambisonics does that literally, which may shed some light on how it allows one to choose whatever pickup pattern one wants after the recording has been made, by clever ratio combinations of the 3 recorded channels which represent omnidirectional pressure, left/right differential, and front/back differential as measured from a single point. 4-channel ambisonics adds a 3rd coincident bidirectional mic oriented in the Z-plane, pointing up/down, to also measure the vertical component, completing the mapping of the entire vector sphere surrounding that single point.