“She wrote me one of those John Deere letters…”

I always knew it as transposing the beginning sounds of two words like: fons of tun instead of tons of fun.

Found some spoonerisms for folk that don’t know what it is -

Three cheers for our queer old dean!" (while giving a toast at a dinner, which Queen Victoria was also attending)[15]

“Is it kisstomary to cuss the bride?” (as opposed to “customary to kiss”)[15]

“The Lord is a shoving leopard.” (instead of “a loving shepherd”)[15]

“A blushing crow.” (“crushing blow”)[15]

“A well-boiled icicle” (“well-oiled bicycle”)[15]

“You were fighting a liar in the quadrangle.” (“lighting a fire”)[15]

“Is the bean dizzy?” (“Dean busy”)[15]

“Someone is occupewing my pie. Please sew me to another sheet.” (“Someone is occupying my pew. Please show me to another seat.”)[15]

“You have hissed all my mystery lectures. You have tasted a whole worm. Please leave Oxford on the next town drain.” (“You have missed all my history lectures. You have wasted a whole term. Please leave Oxford on the next down train.”)[15]

Not necessarily for comedic effect, and it’s for swapping consonants.