That exponential scoring system would work, both by rewarding longer
words and a reason for choosing a higher minimum word length.
Doing this would greatly increase the scores people start outputting.
So you need to do this "now or never". Can't implement this in an
update because then it would essentially wipe out everyone's scores,
which with the new scoring system could have been huge too. Thus, the
complaining.
Have you thought about assigning letters different point values? Sorta
like scrabble. So more difficult letters used like "V" and "Q" are
worth more, especially if you use them AND get the 25 point bonus,
because that is way harder doing that.
So maybe instead of "the score being 5 points * number of letter
position. So the first letter is worth 5, the second worth 10, the
third worth 15." It would be 5 * the total of the added up points of
the word you spelled.
Also, instead of it being 5 * / 6 * / 7 * etc... Have it be 2 * / 3
* / 4 * etc... respectively with the "minimum letters" picked. So if I
choose a 4 letter minimum, it will be a 4X multiplier.