I know its not possible to "excess" out of a TOC specific route (i.e. once your ticket is LM only you can not pay more to make it VT only or any permitted), but is it possible to excess from Any Permitted into a TOC only route (then where your there your stuck with it)?
If we accept that it's written in stone that you are not allowed to excess between 'Any Permitted' and 'TOC-specific' tickets, then the same logic would apply as it's the same change (just in reverse) so the limitations should still apply.
However, if you start from the premise that the reason that it's not possible to excess from the TOC-specific to 'Any Permitted' is because the TOC that set the restrictions isn't willing to grant any additional flexibility (even if given consideration), then the counter-argument would be that in the reverse direction the flexibility provided by the 'Any Permitted' is the customer's and they can give it up if they choose to.
However, I'm willing to bet there's no hard and fast rule since the 'Any Permitted' would allow the passenger to just use the services of the TOC concerned, and most of the time it's the more expensive ticket anyway.