There will be a change in speed by 30 september, according to a note I received in the post today.
More about that further down..

My 1024/512 became 2048/1024 two month ago, and was so for a week until I got connected to another connection point.
(I had some problems during rainy and windy weather and the old connection point was very far away.)
At the new connection, I got 2048/832, sometimes 2048/896, so I complained
The technician explained that double speed was only for the down traffic, something I had to accept but with a lot of doubt especially since I had it on the up traffic on the old connection point..

The note I received today seems to confirm what he told me, here are the figures.
Columns are Subscribed speed, Old double speed, New double speed.

256/128 , 512/256, 1024/512
512/256 , 1024/512, 1536/512
1024/512, 2048/512, 2560/512
2048/512, 4096/512, 4096/512

The table also shows that the highest upload speed is limited to 512Kbps.