# rename files from 20* -> to*:

$ find 20* -type f | time shloop -e rename -s ^20:to
       10.62 real         0.39 user         0.27 sys

# now "mv" them back:

$ find to* -type f | time shloop -e mv -s ^to:20
       18.99 real         0.96 user         6.93 sys