I do not have much luck getting a response from the reseller.  When we did our upgrade to Sage 100 ERP 2015 from MAS90 in December our fixed assets module stopped working.  It took them 4 months to get it working again.

When you look at the chart of accounts it shows the new structure:

5015-00-00

5016-00-00

5017-00-00

Etc.

When I try to make a general journal entry:

General Ledger -->Main -->General Journal Entry

Tab 1. Header: Source Journal = JE, Posting Date = Today

Tab 2. Lines: Account No.  I hit the "search" button and the list of accounts comes up. The accounts show as AAA-AA-AA.  I pick the account I want, 5101-00-00 and the "Account Entry" box pops up.  It asks for the Segment (Main, Sub acct 1, Sub acct2), but this window shows the account number as 5101-00-51.  So I change the second sub account to 00 so the account number is back to 5101-00-00. I hit "ok" and then I get the error "The Account Number is greater than 9 character(s)".

It looks as though we have an account number limit of 9????? Which is the same length of the default account key. I don't know if they are related?? How can I change the acceptable account number length?

I am unable to see the screen prints posted to the forum.  Changes to the account structure do not automatically generate new G/L accounts with that new segment. See KB 20067. It is highly recommended that you test in a copy company first, then make a backup of the live company before making the changes in the live company.  

Were you able to find a way to extend the account key maximum length?

AccountKey has a hard-coded size limit.  Accounts (this thread) do not have that maximum.