Changing One Way to Return (miles ticket)

In order to get the flights we wanted form LGW to MCO for Christmas we decided we would book each leg as the dates (or close to) became available.
Having booked 4 tickets for 16/12 out we then wanted to book the return leg for 7/1 but were told this could not be done as an update instead it would have to be a cancel (25k miles+£100) penalty. This was only an issue because of the 'Miles Further' promotion - a return trip is 40k miles so we would be able to add the return leg for only an additional 15k miles over the one way 'fare'.
We accepted the fact that there would have been a £100 charge for changing the one-way (now I'm just lowly Ag) but £100 to save 40k miles seemed fair.
With that avenue blocked and not being willing to shell out £100 to save only 15k miles we decide to just buy 4 additional one-way tickets for the return leg.
Now ultimately this is never normally an issue as 2*one way = 1 return but I was wondering if anyone had any experience of changing a one way to a return - I guess normally it would be cheaper to go the 2*one way options.
Phil FM
Having booked 4 tickets for 16/12 out we then wanted to book the return leg for 7/1 but were told this could not be done as an update instead it would have to be a cancel (25k miles+£100) penalty. This was only an issue because of the 'Miles Further' promotion - a return trip is 40k miles so we would be able to add the return leg for only an additional 15k miles over the one way 'fare'.
We accepted the fact that there would have been a £100 charge for changing the one-way (now I'm just lowly Ag) but £100 to save 40k miles seemed fair.
With that avenue blocked and not being willing to shell out £100 to save only 15k miles we decide to just buy 4 additional one-way tickets for the return leg.
Now ultimately this is never normally an issue as 2*one way = 1 return but I was wondering if anyone had any experience of changing a one way to a return - I guess normally it would be cheaper to go the 2*one way options.
Phil FM