The chain length issue may be moot. If you have appreciable miles on your current chain and cassette, the old chain may skip and not fit properly on the new cassette. As a general rule, a new cassette requires a new chain.

As noted by Andy and others above; add both spacers and see if the lockring still threads on properly. If it does, use both. If it doesn't remove the 1 mm spacer and try again. Also plan on readjusting your rear derailleur settings. The change in freehub and cassette spacers almost guarantees you will have to reset both your limit screws and the cable tension.