Since they're separate swwitches, you're fine changing the LDN in switch A. That little part was omitted.

However depending on your tieline type, you may run into a problem there... you don't have to make 6000 IDC entries. Only one for each leading set of digits. If all your old NXX are 555-1xxx, 2xxx, 3xxx, 4xxx, for example, then you're just going up to 6 entries. You can do partial DID translation.
Partial Digit Conversion
Not all of the digits received from the Central Office (CO) are converted.
The remaining digits may remain unchanged, and the whole string of digits
is forwarded to the Directory Number (DN) translator.
It is possible to convert a partial string of digits to another partial string of
digits of a different length (for example, 23xx to 4xx or 2xx to 49xx). The
range of DNs to convert can include a mix of DN lengths.
Matthew - Technical Support Engineer Sr.