Does anyone know what happens if you book a ticket that is a one month return and you fly 3/4 legs and then try to move the last leg to a later time I've beyond the original fare rules?

As is often the case here, the answer varies. Based on lack of detail, any answers you get might be wrong.

What sort of ticket? Super cheap? Full fare?
City pairs?

Looking to change BEFORE the original 4th leg, or at a date after "missing" that leg?

Luggage?

You either loose leg 4/4 or they charge you the difference to upgrade the ticket to next category

they will/should upsell the whole ticket (outbound as well as invound) to a 4month/12month fare as regarding the max stay the most restrictive rule will apply

sometimes it's way cheaper to drop leg 4 and buy a one-way than to upsell everything

Minus the 1 month validity, I did that couple years back, SIN-AKL via MEL. While in AU on the inbound I moved SIN-MEL beyond the original validity and paid the fare difference on AKL-SIN, or at least that's what I recall.

Edie: Actually I also did do something like that few years ago, a max stay on a F fare was 3 months, flew outbound then pushed the 6th leg of the inbound beyond the 3 months (back when stopovers in DXB were permitted ) and only paid the difference on the inbound.

Don't know if the processes changed though.

Once the outbound is flown, max stay and penalties of the inbound apply. A complete reprice only happens if the changes are made prior to commencement of travel or during a stopover on the outbound.

the fares does not have to be repriced after the outbound is flown, correct, however the fare rules still apply (combinability, flight application etc etc) so does the min/max stay which is always stricter rule. otherwise you could always buy a 1 month ticket and exchange afterwards your inbound to another date so that your stay is - let's say - 5 months. If so, who would buy 12M tfares where you pay for a 12M fare on in- and outbound.

i agree with you that penalties/changes do apply as per fare component - at least on EK tickets.

Same story if you look at the cancellation fee - also stricter rule, doesn't matter if you have flown the outbound or not (despite that the "cancellation after departure" fee is applicable).
if the outbound fare is non-refundable and the inbound is full refundable and maybe a 400usd fee after departure, the ticket will still be non refundable (theoretically you can take the non-refundable part on an EMD and use it for another ticket but this is another story )

maybe you are lucky and the agent does not get it that the original ticket is only valid for 1M (he/she will notice later when they get an under collection from HQ)

Again, different tickets have different rules.

Until we know more details, the rest is probably assumptive guessing.

Was looking at ex Cmb fares which are now quite restrictive.

You can't stop on the way out in Dxb is you have to transit (can live with that).

On the return you can stop for a max of 7 days and the whole ticket is a one month return. Basically I don't want to go back within a week.