How high are the timbers/bed now? You could just replace them with fresh timbers. I had to move my 8'x4'x12" raised veggie bed this spring. I pried the frame up, moved it to where it needed to be, and then shoveled the soil back into it. Point being, the soil mostly stayed where it was after I lifted the bed frame up, with just a bit trickling down on the sides. You'd probably find the same situation: you remove the rotting timbers, the soil mostly stays put, you lay down and secure new timbers, and perhaps you have to do a bit of cleanup shoveling.

If you don't want to use timbers, you might be able to use boulders/rocks, depending on how high the sides of your bed are. All of my perennial beds are edged with boulders, ranging in size from about 6" to 12" in diameter. It takes a while to figure out which stones need to go next to which other stones for the best fit, but I like the look of the end result. It's not as easy to weed-whack between the stones, however. I usually just end up hand weeding while I'm tending to the rest of the beds.

Timbers are probably cheaper, unless you have a place where you can get cheap stone. I dug most of my boulders out of my yard, and bought the rest at a local hardscape supply yard for $35 a wheelbarrow-load.

Can you add a photo so we have an idea what you are talking about? 6" or 2' or?

Here are some pictures

Thanks! It would certainly be easier to just remove them and make them flush. About how much do you think we should add to the depth? They are about 3 feet from front to back on the side of the house, and 4 feet in front.

Also, it is aluminum siding. Does that change anything as far as the concern over rot? I can't tell if there's wood directly behind it...

If grade is heaped up on aluminum siding, it's still a bad thing. It is what is behind the siding and inside of the wall that can be affected. Having soil and mulch in contact with siding not only invites moisture, but it invites insects, some of which can cause a lot of damage.

I would consider 4' AN ABSOLUTE MINIMUM for bed depth ... mostly good for perennials and annuals, but not so much for shrubs. Keep in mind that there should be some space between the plantings and the house siding. A more acceptable minimum is 6'. 8' is not ridiculous. Is is normal considering the actual size of typical foundation plantings. Keep in mind that planting a single row of a given plant is not an ideal. It is a stingy minimum. If Hostas look good, a bed depth that accommodates 2 or 3 rows of them looks even better.

I am with Yardvaark on pretty much everything he said. My foundation plantings tend to be 6-10' deep, with usually about a foot behind the nearest planting to provide access to the building for maintenance and to allow things to dry out. If you decide to use a brick or patio block mowing strip, also use a buried edging strip to keep grass from growing into the beds; the brick alone won't keep out grass. Your plants look full enough so that this fall when the weather has cooled, you can dig them and both move them outward a foot or so, and divide them to provide relatively full beds next spring. If you want to get started sooner, you can remove sod and install the edging over the summer, using mulch to keep weeds down until you are ready to replant.

I agree with Yardvaark, too. Your beds look tiny, especially as related to how many plants you have there.

I like to use a trenched edge. It's clean-looking and cheap. Here's the tool I use to create it. Just stand on it and cut the soil in a long wedge. If you're careful, you can pull out the soil-wedge in long strips. Easy.