The impacts I could see are:
1) User training. Users hate change!

2) Any phantom 500 or ACDN that are used to route a 4-digit extension out to an external number
3) Again with users, anyone that uses external call forwarding would need to change from 6+ to 4+
4) Any voicemail menus that xfer a call externally
5) Voicemail restriction / permission lists
6) Symposium / Contact Center scripts that route a call externally
7) ??? maybe more?
To be quite honest - changing one of your BARS/NARS access codes is a big undertaking that could have a lot of impact.
Might I suggest (although it kind of muddies the water...) that you use a different range for this new site? Even if the new site you are networking to already has 6xxx extensions, you could still dial (for example) 4xxx from your site and translate it to 6xxx as it leaves your PBX.
To do that, go to LD 86:
REQ PRT
CUST 0
FEAT DGT
DMI <enter>
You should (hopefully) have an unused one - only 0 thru 10 are going to print (based on your previous printout of the ESN datablock). If you see all 10 used, then do:
LD 86
REQ CHG
CUST 0
FEAT ESN
<enter thru to MXDM>
MXDM 20
<enter thru to end>
Then also in LD 86:
REQ NEW
CUST 0
FEAT DGT
DMI <insert unused number from 1-10 or 1-20, depending on if you had to change the MXDM>
...
DEL 1
INST 6
CTYP NCHG
<enter to end>
Sorry, I don't have my NTP's open so this is from memory; my apologies if the prompts are out of order.
This will basically delete your first digit (ie, if you use 4xxx then it deletes the 4), and then inserts a "6" in front of the remaining digits.
Thus, since your far-end site is already using 6xxx extensions, even though you dial *4xxx* from this site, it translates to *6xxx* and your call completes fine.
I dislike this in a large network, because it creates programming confusion - it's always best to plan and coordinate your dialing plan network-wide, but sometimes we have to make programming compromises for the ease of our users...
I know I just threw a ton of info at you - hope I wrote it clearly.
~M~
Matthew - Technical Support Engineer