a bigger problem will be changing the whole tuning between E and D on the same bass; you ideally want thicker strings for the lower tuning, and to adjust the bass to play right in that tuning. changing between E and Eb is probably close enough to get away with, but bigger changes warrant different string gauges and setups.
aside from that, i can drop my E string down to D in about 5 seconds, not 30; what kind of tuner are you using?
Guitar Repair Gnome
Alpha Music, VA Beach