The longer the tree continues to lean, the more difficult it will be to bring the tree back in line with vertical. Sometimes these trees pull out of it; however, sometimes we need to jump in and help the tree recover its original shape.
Corrective measures often include guy lines and/or staking as you indicated in your original questions. Selective pruning may also be necessary. As always, it is best to consult a certified arborist as on-site consultation is usually superior to on-line advice :-)