Answer: I think the main concern when repotting your lemon tree is to keep from stressing it so it doesn't drop the fruit. Since you're in Portland, I'll presume your tree is indoors for the fall and winter months and outdoors in the spring and summer. It should already be indoors, and hopefully it has adjusted to being inside. If not, move it in now and wait a month or two before repotting, just to make sure it makes the transition to growing indoors without stress.
As for repotting, it's always best to use fresh potting soil and to move it into the next size container. If there's too much room in the container, the soil will sour before the roots have a chance to venture into it. For example, if your tree is in a 5-gallon container, you can move it up to a 7-gallon. If you have a much larger pot in mind, such as a decorative pot, you can still move the tree up to the next sized nursery pot and then simply set the plastic pot into your decorative pot. Good drainage is essential for your tree so check the new pot for adequate drainage holes.
Here's the procedure for transplanting: moisten the potting soil and fill the bottom of the pot with several inches. Too check for depth, set the current pot (tree and all) into the new one to make sure that the tree will be planted at the same soil level as it is now. Once you're satisified that you have just the right amount of soil in the bottom of the new pot, take a long, slender stick (or a knife) and run it all along the inside of the pot to loosen the soil. Then (with help), turn the tree and pot on its side and gently tap the bottom of the pot. Slide the tree out of the old pot and set it in the new pot. Carefully comb the sides of the rootmass with your fingers to help loosen the roots (so they'll grow into the new soil), then fill all around the sides of the rootmass with moistened potting soil, gently tamping it down as you go. When you've finished filling the pot with soil, water it well to help settle the soil.
Keep emptying the saucer as excess water drains from the soil - you don't want the roots sitting in a puddle. Place the newly transplanted tree in its regular spot so the light, temperature, etc. is the same as it was before the transplant.
Using the above method should result in a stress-free move. Don't fertilize your tree for 6-8 weeks. By that time the roots will have had time to recover and will be able to use the nutrients you provide.
Best wishes with your lemon tree!
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