The first thing to do is remember that all tables are partitioned. Sort of. What we think of as non-partitioned tables are really just tables with a single partition. Every table is listed in sys.partitions and in fact you can use it to quickly see how many rows there are in a table. Since there is no partition scheme/function we can’t do splits or merges, but we can do a SWITCH.
What we are going to do is create a new, virtually identical table, then switch the data from the old table (partition) to the new table (partition). The trick is that while in order to do the switch almost everything has to be identical, the properties of the identity column are part of that almost.
I love these types of solutions: hacks in the most positive connotation of the term.