Muskets are Worse than Longswords (only cheaper/no iron), and
Usually worse than Crossbows (stronger, but no Range)
Usually worse than Pikemen (stronger, but not v. Horses)
You need Riflemen.... Those are better than Longswords, Better than pikemen, and usually better than Crossbows (no Range, but >2x strength)
So you can't "downgrade" your longswords to muskets, but you can upgrade them to Rifles.