The “classic” style forum would work perfect on a smartphone with the right theme.. which many are available for free. This one is alright, but nowhere near as good as the old forum was.
Switching forum software should not have been a financial decision. phpBB, which is what the old forum software was, is completely free. The old forum used an older version of phpBB and if I remember correctly its been said there was some reason the old forum could not be upgraded to the latest version of phpBB. Even so, they still could have created a brand new install of phpBB.