According to a recent issue of WORD WAYS, one of the most difficult feats in English verbal acrostics is to take a word spelled with a "U" and to change that "U" to a "V," producing an equally legitimate word. The only common word-pair having this property is VALUE-VALVE: Marty Fingeshut suggests the rarer pairs LEU-LEV, MOUE-MOVE, TAU-TAV and ROUE-ROVE.
Eckler, A. Ross
"Word-Pairs Differing in a Single Letter,"
Word Ways: Vol. 2
, Article 4.
Available at: http://digitalcommons.butler.edu/wordways/vol2/iss2/4