@JL
[2014-02-14 18:23:55.255] Database is at level 13
[2014-02-14 18:23:55.267] DEBUG: Will apply sql:
ALTER TABLE block DROP COLUMN IF EXISTS index
[2014-02-14 18:26:26.984] DEBUG: Will apply sql:
ALTER TABLE transaction DROP COLUMN IF EXISTS index
[2014-02-14 18:33:32.807] Updated database is at level 15
Didn't the additional indexes help?
Question: Is it possible to re-use the nxt_db directory when upgrading from 0.7.4 to 0.7.5 without downloading the whole blockchain again?
Raspis do have a tough time doing that, crashes 1-3 each time, must be restarted, takes severla hours in total...
No, no.

It does not. Just overclock it at medium level. Works like a charm for me.
And yes, you can re-use the db of 0.7.4.
In fact the nxt.log above shows the migration from 0.7.4 to 0.7.5.