If the mere act of filing the application negates the grandfathering, then I'd never take the shot.
If it does not, I'm still not sure I'd do it, as I potentially question the economics. Let's say it allows you to build 20 more spaces by doing so, then you'd have $350,000 in alterations to current park, and around $20,000 per space in construction and pad preparation (this depends on your topography as well as installation requirements), so you'd have $750,000 invested in those 20 spaces. If your lot rent was $300 per month and the park pays water and sewer, each occupied lot is worth $21,600 at a 10% cap rate, and you lost around $350,000 if you sold the park the day after building those lots. To expand a park successfully, you need zero cost from the existing property, easy topography and installation cost ($15,000 per lot or so) and high lot rents ($400 and up). But you may have those things and, if so, disregard what I just said.
Out of over 250 parks we've collectively owned over the past 20 years, we have only expanded probably 10 to 15 of those.