I think you would have to use a manual crosstab to accomplish this. There are FAQs about how to go about this, but basically you would group on your row field, and then create a series of formulas for your column fields, as in:
//{@Jan} to be placed in the detail section:
if month({table.date}) = 1 then {table.amount}
//{@Feb}:
if month({table.date}) = 2 then {table.amount}
Then you would insert a summary (sum) on each of these formulas and suppress the detail section. Let's say your group is based on {table.ID}. Then to get the change from the previous month, you would create a formula like:
(sum({@Feb},{table.ID})-sum({@Jan},{table.ID})) %
sum({@Jan},{table.ID})
-LB