I get asked this on a regular basis so I thought it time to summarise our thoughts....
Ideally we only need the lid as it's this part of the frame to which the pane is sealed and the closing seal attached. The closing seal is integral to the pane, so normal practice is to replace both at the same time.
Old hatches will have corroded fasteners, which more often will resist removal. Using a heat gun (ideally with a narrow nozzle), some releasing fluid (even white spirit) and a hammer/drift to shock the corrosive bond between fastener and surroundings are good techniques to aid removal and should be applied in advance, before you burr over the screws. It also goes without saying that if using a screwdriver, make sure it is the correct size so that the fit is exact and tight. Small mole grips can also be useful for holding a nut/bush.
The pane is sealed into the top of the lid frame. The lid is connected to the base frame via hinges. The upper hinge has a strap which is held on with two recessed bushes and M6 machine screws.
You have four options, which in order of best to worst are;
Remove lower hinge part from base
Remove whole hatch
Remove upper hinge part from acrylic
Disassemble hinge by removing hinge pin
Remove lower hinge part from base - The best option - Each lower hinge is attached with two fasteners which can be;
machine screws and captive nuts
machine screws and non-captive nuts
Of these, the first two are easy, assuming they can be undone. If the nuts are not held then you'd probably need to remove the head lining to get a socket on which may not be easy, depending on the craft.
Clearly, your first task is to determine which method of attachment has been used, so try removing one of the fasteners. If it turns and starts to rise you're in heaven, so a big sigh of relief. If it turns but stays put you have non-captive nuts so a measured curse is appropriate and then investigate removing the headlining.
Remove whole hatch - a common option where you find your fasteners can be removed. As above, test a few fasteners first. If they rise and can be removed this can be a good option as it means your upper hinge units will get resealed by us to the new acrylic. If original it's also likely that the bedding compound will be getting crusty, so re-bedding the whole hatch may be a prudent course anyway.
If the fasteners don't rise then evaluate how you can access the securing nuts from below. If feasible, this remains a good option. If difficult, move on to the next option.
Remove upper hinge part from acrylic - removing the upper hinge straps from the acrylic leaves the hinges attached to the base, but means you'll have to reseal the straps on the acrylic yourself. The original fasteners are chromed brass so softer than stainless steel. The bushes have a slotted head which may well be corroded and is vulnerable to splitting during removal. Try to unscrew with a suitable screwdriver. They may release, it happens. If not, then rather than using the slot in the bush to twist the bush we suggest using the small mole grips to hold the perimeter. This will give better leverage, apply pressure evenly and avoid potential splitting. Hopefully this will yield some movement. If nothing works, use a chisel to force the top of the bush away (this will be quite easy) and then use a suitable drift and hammer to push the remaining shaft through the acrylic. Then hold the shank in a suitable tool and unscrew.
We hold a stock of replacement stainless steel M6 bushes (and machine screws) as replacements for the original bushes. These are not slotted, but have a hex key head.
Disassemble hinge by removing hinge pin - This really is a last resort and best avoided as spares are almost impossible to find. Lewmar stopped producing hinge kits many years ago, so the most likely source is now fellow sailors finding kits they had hidden away and now want to pass on after selling their boat. Ebay is an obvious platform to find these. There is also a company in the US (hatchmasters.com) that has arranged for some models to be re-manufactured, but last time I looked most were out of stock.
We do have some of the Lewmar guidance notes (supplied with kits) which would aid removal, but as mentioned above this really should be a last resort.
Similar to Superhatch except that the upper hinge unit is integral to the lid, so removal is not an option. Same comments apply to other options.
Rollstop size 30 and smaller have a rivet (as well as two fasteners) holding each lower hinge unit to the base. As a result, removal of the whole hatch for these sizes may be the best option, unless you want to drill out the rivet. Lewmar say the rivet was just to hold the frame parts together during shipping, so replacement after removal is not required.
Hope this helps...