When booking you can definitely do as you describe. There's some extra taxes to pay. You can use the "multi-sector booking" online to check the prices.
To change it after ticketed will depend on your ticketing rules and there's probably change fees if it's a lower Y class. So you can only find this out by calling reservations and asking for the costs to change.
Thanks for this. We will be in PE and it will be the ticket class that lets you change dates etc. free, subject to any fare adjustment. We will book a conventional itinerary, with quick connections both ways, so my concern was if there was a "maximum connection time" that might stop us trying to break the return trip for a few days in London....