My impression is that while existing property generally is grandfathered when regulations change, renovations then require you to bring the relevant part up to code. Thus, if you want to change stuff, like putting up a fence, then it does not seem far-fetched that you would then be required to comply with current public access rules.

If you have an old deal, that's fine. But you can't justifiably claim you both have a right to be grandfathered in while at the same time updating the property to current standards.