In Photoshop all mods of the layer mask are destructive, but it's possible to make easily a spare and restore it if the modification went bust. The next method works with pixel layer masks.
So, here we have only solid green in the image layer and the layer mask makes 3 separate areas visible:
The mask occurs in the Layers panel, it's in the Masks panel and it's also a channel in the Channels panel.
You can make a spare by duplicating the mask channel:
The spare is only a dummy channel but you can restore the original layer mask with it if needed. It's a good idea to rename it for ex. SPARE Mask
Layer mask can be "Refined". You can feather (=blur) and move the mask edge to expand it. Contrast affects the BW contrast of the mask:
You can access the refinement dialog in the Masks panel or in the Layers panel by right clicking the mask icon.
You see the result as you adjust. Checkerboard is replaced by white during the edit.
Here's the effect of the mask edit:
If the original mask must be restored, simply Undo. If undo is no more available, delete the wrong layer mask. Right click it in the Layers panel and select option Delete.
Ctrl+Click the spare channel icon in the Channels panel to create a selection. Then insert a new layer mask. It's based on the selection.
Nothing prevents you to save also a copy of the edited mask as a dummy channel and restore it if needed later.
Other maybe useful things to know:
- you can have an empty layer with a layer mask
- you can move a layer mask to another layer in the layers panel
- painting manually to layer mask is possible, only select the mask icon in the layers panel.You'll see the effect, not the mask.
- you can get a layer mask to onscreen for edits and cut&paste by clicking the mask icon in the layers panel and holding Alt key at the same time, no need to go via the channels panel.