I wasn't working that day but I'll try and answer.
Why you were switched? I don't know, maybe a lighting issue or ILS unserviceability.
We land on one and depart on one in low viz because that has a higher capacity than landing and departing on both due to spacing requirements on approach in low viz. Arrival to arrival in Cat3 is about 8 miles in trail. Put a departure in that space and it needs to be 15 miles. Departures can go in sequence without waiting for arrivals if they have their own runway. So even if you landed and departed on 2 runways the arrival rate would only be about 20-24/hour with a similar departure rate. Land on 1 runway at 8 miles in trail and arrival rate is still about 20-24/hour. Departure rate is over 40/hour on dedicated departure runway even in Cat3. 06R/24L is not useable in these conditions.
Tower frequency change signs were considered but for some reason not adopted. There are a lot of confliction points on the way to all the runways except 05 and so their advantage was deemed to be negligible.
95% of the time Departure will be 128.8. 127.57 will only be used from 05/23 at busy times or occasionaly when departing from 06L/24R and going north or west.
Lester 7 and 3000'. What type do you fly? Do you get turned by tower? Do you depart from a runway different to clearance initially given due direction of flight? If you are departing at the same time as someone from the other runway who is departing from the "wrong" runway then you too get 3000'. Departure will then sort out the crossover.
Thanks for the positive comments.
DC