Okachimachi > UenoOkachimachi

I'll need to transfer from and to the stated stations as in topic. Is this a long walk and is it clearly signposted? General directions would be very much appreciated.

Also I'll be carrying around a 100 litre rucksack, will there be enough space in the trains? also consider that I'll be making the journey at around 4-5pm weekday.

anyone?

Its signed as an interchange yes. As for long walk it would depend on which end of the JR station and train you got off, since you know platforms are long from one end to the other. Part of the JR station is on top of the Toei Station, so it is marked and that walk from station to station isn't far.

I often change trains at Okachimachi station. Toei UenoOkachimachi station is near North Exit of JR station.

Out of the wicket, walk toward MIDORI NO MADOGUCHI (ticket office) and you'll see a drug store in front of you. Turn right and 20-30 meters forward, you will see the Toei entrance on your left hand. Use the elevator to the basement floor. Stations of TOEI OEDO line are generally very deep.

This line (TOEI) is not very busy and 4-5 pm is still early for evening rush hours. I don't think you have a problem carrying a big backpack.