Changing the length of a garment is actually fairly easy (at least conceptually). The technical details are in a previous TECHknitting post called "length reassignment surgery." The purpose of this post is really only to provide an updated link for folks (and they are many) who have not been able to find the surgery link through a search-engine search: the title of THIS post is directly on point, whereas the title of the surgery post was not very findable.
This is the second in a series on garment correction. The others in this series include
Part 1: My sweater is too wide
Part 3:My sweater is too tight under the arms/at the bust/chest--the magic of gussets
Part 4: My hat is too loose
Part 5: My sweater slips off my shoulders
Part 6 (still to come): My sweater is too small around my middle