You can try search saved password in the database (Database editor -> Settings -> ICQ_1 -> double-click "Password" setting)
Easy way to migrate contacts from one account to another:
- go to Account manager
- delete new ICQ account from Miranda
- click on original account (the password from which you lost) and replace old "ICQ number" and "Password" by new ones
Maybe you also should request auth from every contact, because they have not authorized your new number. I just checked - it worked (after the contact authorized me, and I authorized it, we see each other online).