In the context of the text extract that you have quoted, I would interpret the words 'will' that you have highlighted to be a description that explains an instruction that is issued to pilots in some other regulatory document (I hope that makes sense). In reality, it sounds like it was written by an ex-controller who has forgotten who the audience of the document is and also forgotten what the status and purpose of the document is.
In fact this is the only document that tells my to set ONH immediately after receiving an instruction to descend to an altitude. While preparing to fly to UK I opened the Jeppesen ATC section (which is an excerpt from the UK AIP) and read this rule. That's it. It is contrary to ICAO standard and my company procedure which both say to change altimeter setting when actually at transition level/altitude. Here comes the meaning of "will". And with respect to the native speakers I may expect that they mean something when they write and publish it. I'm sure most of the pilots whose mother tongue is not English have serious doubts how mandatory "will" is. From one hand it's a rule, so it seems I have to follow it. From another - why not to make it clear with "shall"? So, I may reasonably think that there is an underlying meaning.