Move data from a table to another without losing data
Consider creating a new table with the partitioning strategy you want, and add a view atop both tables that does a union all. Have people use the view, and write instead-of triggers against the underlying tables &amp; views.

Inserts should be sent to the new table, updates should move data to the new table, and deletes should be applied to both tables.

Then do batch moves in the background, moving as many records at a time as you can over to the new table. You can still have concurrency issues while this is going on, and some terrible execution plans, but it lets you stay online while the moves are happening.

Ideally, you start the process on a Friday afternoon to minimize the effect on end users, and try to get it done before Monday morning. Once it's in place, you can change the view to point to just the new table, and the terrible execution plans go away. Ideally.

To avoid the triggers firing when the data is being migrated in batches, look at the number of rows in the deleted/inserted tables in the trigger, and skip the activities if they're near to the number of rows in your batch.

The more transparent you want this to be for your end users, the more work (and testing) it's going to take. This especially holds true if you're using partitioning: very often folks believe it's going to make all their queries faster, and yet some of them end up much slower. Try to test as much of your workload on a development server with the partitioned tables if you can.