Can you heat up the liner a second time...
Can you heat up the liner a second time once the boot has packed out and remold the liner to your feet? If so how?
You should be able to reheat the liners up to five times without trouble.
You'll want to use a convection oven to make sure the liners heat evenly. Preheat to 250º. Place the liners in the oven - I've heard on a foil-lined cookie sheet, but I've used the bare rack without issue - for 10-15 minutes. You want them to look puffed up like a marshmallow.
While they're heating, prep your feet. I like to use the thinnest sock possible. Put a toe cap on under the sock. If you don't have toe caps, you can make them out of whatever you have around the house. The point is to spread your toes a little bit to give them room once the boot cools. I've used cotton balls and packaging tape.
Also, grab a 2x4 out of the garage and put it on the floor.
When the liners are heated up, pull them out and shove them into your boots, being careful to get the tongue placed properly. Toss your footbeds in there and put the boots on. Strap them one step looser than you normally wear them, kick your heels to get them set into the boots, and stand with the toes of the boots on the 2x4 for about 10 minutes. At the end of the 10 minutes, pull your feet out, strap the boots back up, and let them sit until they cool entirely.