The bodies have published details of their single proposed Code, which they say is "near-identical to the current APPC Code".
They say the changes - announced amid controversy over the plan, with a significant number of senior figures opposed to the merger - are mostly limited to terminology.
For example, references to "the APPC" will become references to "the Public Affairs Board". "The process for investigation complaints and the mechanism for taking appropriate disciplinary action remain independent and unaltered," the groups said in a statement.
They say no changes can be made to the terms of merger before or after APPC members vote at their extraordinary general meeting in October.
Paul Bristow, APPC chair, said: "Members deserve nothing less than thorough detail: the APPC is heading into this merger from a position of strength, and this new Public Affairs Code protects and promotes the gold standard in public affairs while enshrining the independent complaints process. Members will be reassured that the current APPC Code is the basis for this unified code."
One proposal is to re-word the Code's remit to ensure it is not limited to UK-based institutions of government. The current APPC code covers the UK only.
The APPC's Mark Glover said the plan would allow the new Public Affairs Executive Committee "to address examples of bad practice by regulated agencies with foreign institutions".
"This would allow a repeat of the justified action taken against Bell Pottinger, which the APPC Code by its limited scope was restricted from addressing. It also tightens the rules around employing sitting British MPs and peers engaged in public affairs activities abroad. Which in my view would be a welcome tightening of the code."
However, twice former APPC chair Michael Burrell said: "Extending the code to the whole world may or may not be the right decision. I make no comment on that. However, I was very unhappy to see such an important announcement about how the ‘merger’ would operate being made without any prior consultation with the APPC management committee.
"In the past all such code changes and their practical implications have been extensively debated by the committee before being made. Now we are all presented with a ‘take it, or leave it’ decision in which the committee has had no say. I am deeply concerned that this undemocratic approach to decision-taking may be a taste of things to come should members vote in favour of the PRCA proposal."
Darren Caplan, CEO of the Railway Industry Association, speaking on behalf of the campaign for an independent APPC, said: "Further details regarding the proposed takeover are welcome though should have been published weeks ago. That said these details will convince none of those who support an independent specialist body for public affairs practitioners that their interests will be better served in a large commercial trade association.
"This proposal is also the final proof, as if it were needed, that this is a takeover and not a merger, as all mentions of the ‘APPC’ disappeared in the Code of Conduct to be replaced by a PRCA 'Public Affairs Board'."
'Details set in stone'
Meanwhile, other proposed changes to the PRCA’s Memorandum and Articles of Association include a clause automatically making the chair of the Public Affairs Board a member and officer of the PRCA Board and amendments to guarantee changes to the Public Affairs Code can only be made with the majority support of both the Public Affairs Board Executive Committee and the PRCA Board.
Ingham said: "Today, we are publishing the final pieces of detail, showing precisely how the new Public Affairs Board that would be created by an APPC-PRCA merger would work.
"Three separate documents set out: the new unified Code; the new unified disciplinary procedures; and the formal changes to the PRCA Memorandum and Articles of Association that lock in the new Code, the new disciplinary process, and the autonomy and status of the Public Affairs Board. In addition, I have also signed the MoU on behalf of the PRCA Board.
"In doing so, our aim is to leave nobody in any doubt about what they are voting for. This detail is set in stone - unamendable by the PRCA. It is now for APPC members to vote on merger from a position of absolute certainty on the shape and governance of the new body they are being asked to create alongside us."
Darren Caplan's quotation was updated on Friday afternoon to clarify that the name of the proposed board will be the "Public Affairs Board", not the "PRCA Public Affairs Board".