I don’t think that there is a way to do that without using any custom SQL code.
If you have access to the database, you can change the latest sequence number back to what you need. But I don’t know what happens to existing data that is using a number higher than your new starting point. Be carefull with that.
Increasing the new starting number is easier. You can just increase the default value to your new starting point.
I don’t think it’s possible. Do you allready have entries in your DB for this entity?
If you would create a new autonumber attribute all existing entities will get numbered in sequence again in this new attribute. You could then delete the first attirbute and rename the new attribute to the old name. But you will get a different order number for your eixisting enities offcourse. But the numbers will be in sequence again.