Contrast will change.(posted 9365 days ago)What happens is that as the highlights in the negative receive fresh developer, they will develop faster than the lows. The same effect would be achieved by keeping agitation and time constant, but raising developer temperature. Or using a stronger developer dilution.
Back around the turn of the century, there were a number of photographers who photographed the interiors of cathedrals. The speed of the plates they used was so slow and the light was so dim inside the churches, that they used highly dilute developers with a development time of something like eight hours. Oh yes, and very little agitation.