Dunno about wechat but whatsapp will keep using the number you first activate/register it with unless you go change in the settings section. It doesn't actually actively check your current phone number... even when change it just send an activation text to your new number and if received correctly it switched to that.

I actually kept using my old number as my whatsapp number for a year despite new sim and number until I got unlazy and changed it, lol.