Answer: Improving the soil is so important and so beneficial for the plants in the long run that it is worth the trouble -- you will have to lift the plants, incorporate the amendments and then replant. (You can't raise the soil level around existing plants -- they will smother for lack of air at the roots and/or the crowns will rot.)
The plants should be dug up with hefty root balls and should not be out of the ground any longer than necessary -- protect the roots from temperature extremes, drying sun and wind and place them in the shade. If you think it is going to take you a while to accomplish the work (more than a day or so), you might consider designating a holding area where you can set the plants into the soil temporarily.
Early spring is a good time for transplanting. Take a large root ball, replant immediately at the same depth the plant grew before, firm the soil gently but firmly around it and water it in. Most plants won't know they've been moved if you work carefully.
Finally, keep in mind that once the soil in your perennial beds is well prepared, then regular mulching with organic materials and occasional top-dressing with compost or shredded leaves or well-rotted aged manure can all help to build and maintain the soil on an on-going basis. Your plants will definitely show the difference, too!
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