If your income hasn't increased or expenses decreased I can see no reason that the creditors would ask for an increase in payments or extension.
A lot will depend upon how the variation is presented so as not to give the creditors that idea or opportunity.
As long as GT don't try to over complicate things and simply present a variation in these terms:
Motion 1: that the creditors accept the lump sum as proposed in full and final settlement of the arrangement. Or,
Motion 2: the creditors reject the lump sum offer and the arrangement continues as originally agreed.
I would emphasise that this is my untrained line of thought and it would be prudent to await an expert's opinion

My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014