Q:

Changing entry_id: possible, but asking for trouble? 

We have an internal system for assigning three digit numbers to the various accounts where we accept donations.  These are set in stone and cannot be changed.  If we could match these three digit numbers to the entry_id that would make a several things better:
-url would have the account number and not a different number when donating
-carthrob uses the entry_id to send info to the gateway, but that is meaningless to us.  If it matched our account numbers that would be very useful. 
I know I can go in and muck around the database and change these, but is that asking for trouble?  What happens if we have 300 entries and we've set one of these accounts with an entry ID of 301?  Will EE skip 301 or create two 301s...or just crash in a blaze of white screen? 
THe site will be live tomorrow or Friday.  It is not very content heavy.  Maybe 30 pages, 20 members right now (using Zoo Visitor so they are in a Channel),  and the 75 accounts stored in the CartThrob product channel.  The entry ID count is nearly 300 with all the test orders we've placed to test the donate function.  
Anyone do this and want to talk me off the ledge?  Or is it safe to jump?  

A:

That's very much asking for trouble.  I recommend just adding a new field to the channel or to store the internal id. We do this for a number of clients who need information to correlate to some other database or system.  You can create a field called: account_number and use that and it will store the data safely. 
If you adjust the entry number manually, you have to adjust it everywhere, titles, data, category, member_id, etc.  Don't do it. 
Custom fields are your friend.