Well Bookworm, there is a blurry line between Sidestep manoeuvres and Circling approaches, but it is important to try and differentiate one from the other so that one may execute the correct GA.
There are only three sorts of approaches for IFR flights contemplated by PANS-OPS and PANS-ATM:
1) A straight in instrument approach where the landing runway is the instrument runway
2) An instrument approach followed by visual circling where the landing runway is not the instrument runway
3) A visual approach
There is no such thing as a "Sidestep manoeuvre". Your action in the event of a missed approach will depend on the nature of your clearance.
1) If the clearance is for an
instrument approach for the new runway ("cleared ILS approach 27L") then you fly in accordance with the new IAP. This is clearly only possible if the landing runway (27L here) is an instrument runway, which is not, as far as I know, a prerequisite for the manoeuvre that hetfield asked the original question about. It is hard to see how such a clearance would be offered and/or accepted at anything other than range long enough to allow the crew to rebrief. I don't believe the "swing overs" under discussion fall into this category.
2) If the modification of the clearance is to land on a different runway, I can see no difference between this and a visual manoeuvring (circling) procedure, and the missed approach for that procedure sould be followed. Spitoon and I have already quoted that, and it consists, ultimately, of flying the missed approach for the original instrument runway.
3) If the modification of the clearance is for a visual approach to a different runway, there is no provision for an instrument missed approach. The aircraft is expected to remain visual.
According to Intruder's description, the FAA defines a fourth category, the sidestep, which appears to be substantially the same as visual manoeuvring (circling) but possibly with lower minima.
Either local procedures (e.g. EHAM) or specific instructions from ATC can override these ICAO defaults. Heathrow MATS Pt 2 clearly requires the latter, an explicit set of instructions to follow.
While it is undoubtedly prudent to use one's experience and awareness of ATC procedures to build an expectation of what may be required by explicit instruction at particular airports, in general the PANS-OPS procedures need to be followed, and I think that's what hetfield was asking.