This is not so easy, or maybe it is.
Create the new band with the BandMath operator as usual.
As next step you use the Subset operator to create a product without the band to be replaced.
Afterwards you merge the both results.
I've attached an example graph which shows how it should work.
Unfortunately you have to name all bands in the subset step except the one you want to replace.
In future releases it will be possible to define excludes in Merge operator.
math_subset_merge.xml (1.4 KB)