I would think right about now would be the time to repot, just before significant growth begins for the season. I wouldn't use potting soil on its own. I think a blend of three different kinds of soil is a good idea. In my garden I often do sort of a random blend of my native soil (perhaps 50%) with additional commercially packaged humus, compost, topsoil, potting mix, etc. (10 to 25% each), and I have had good results with all different sorts of fruits and veggies doing this. Diversity seems to be the key in successful gardening. After your plants have been growing for 4 to 6 weeks, you can start feeding nitrogen fertilizers, which will help keep the plants very green and vigorous. Then once late summer / early autumn hits, cut off the fertilizer to allow the plants to fruit and then go dormant.