I was just cruising through this forum and saw your question, which no doubt you have found the answer to. Let me say that two things have worked for us on the Definity PBX when dealing with 8400 series sets and the softkey translations. First, we can program a 'Refresh Terminal Parameters Access Code' in the 'Change Feature Access Codes' form. From a set with console permissions, you can dial this code, followed by the 8400 series extension, and you'll hear the 3-burst confirmation that the download has been completed.
The second way is to busy the 8400, make the changes, then release the set.