How long should I wait before watering my plant, causing the newly added soil around the stem to get wet? The old soil underneath still has some moisture.

Stem rot. Or dampening out is a real threat! I would worry about this—- not too much water

Hi—- hard to say because we can’t see what’s going on! Stick your finger in the soil and check for moisture. Pick up the pot—- super light—- give a little water. Unfortunately this is every tactile. Good luck ✌️🏻💚🌿💨

Regardless of your incident, When you transplant some thing into a bigger pot do you want to water through so that the two soil’s become one otherwise what happens is the next time you go to water it will not saturate down into the soil that was originally around the plant. This prevents dry spots in allows your roots to go where needed 🌱

I personally do an entire watering & saturation of the soil when I do a transplant like this.. I wouldn't be too worried about the stem rotting. That dry soil with fans and air movement makes a huge mess over time in my experience.. Plus the back of the soil I use suggests doing a full saturation and watering immediately after transplant - so thats the way I've always done it when using soil.