I've heard of the "roll it onto the grass" technique, but I've never done it personally. Getting it off sounds straightforward enough... it's getting it back on that concerns me. Seems like the hitch end of the trailer will end up pointing skywards unless you hitch it to a tow vehicle, which then means the boat has to climb up onto the rollers by itself, which is something it was probably never designed to do. Last thing you want is to crack the keel or pull out the towing eye.
When I replaced the axle on my first boat I just jacked the trailer up (including boat) then blocked it off and dropped the old axle. I didn't use a grinder to remove the old bolts - mine came off with some penetrating fluid and a breaker bar with extension (aka a 3 foot length of scaffold pole slid over the end of the breaker bar). If I were to do it again I would investigate this route first. Even if you find you need to grind off the bolts I'd still do it with the boat on the trailer. You should be able to access the bolt heads from above the axle, thus avoiding the need for grinding from underneath and risking sparks, swarf or spinning grinders falling on you.
One other suggestion - you can buy nut splitters quite cheaply. Could be a good (read: safer) alternative to grinding them off. https://www.toolstation.com/nut-splitter-set/p80007