You can change at Reading at the moment for Waterloo if you prefer not to use the underground. Takes a little bit longer though.
Many people vigorously claim that nobody will want to change at Reading for Crossrail when they could stay on a faster train to Paddington - then switch to Crossrail.
I don't believe this will be true in practice.
Reading to Crossrail will be a straightforward platform change - people now have the data at their fingertips to be able to make the connection choice too. It will also save trekking around Paddington.
Whether people change at Reading for Crossrail will depend on their individual journeys and their own preferences, but they should consider the situation as it will be.
On the basis of current planning, off-peak there will be two Crossrail trains per hour between Reading for London. These replace the two Reading terminators on the Relief lines. Essentially these call at all but a couple of the stations en route.
The Crossrail web site shows journey times to Paddington of 50 minutes compared to the fastest current (Intercity) time of 28 mins. However it also lists times to Bond Street and Tottenham Court Road which by Crossrail will be 51 or 52 mins and by the current service are 53 or 55 mins. but these times are by the existing Underground routing.
In other words after the expected acceleration on completion of engineering work and the introduction of the Hitachi trains on many services the fast trains will be some 30 minutes faster to Paddington than Crossrail.
There will be, off-peak, some 9 or 10 non-stop trains to London which call at Reading. This means that it will be about 30 mins faster to places like Bond Street if one remains on these trains and then changes to Crossrail at Paddington. The access to Crossrail under Eastbourne Terrace will be easy as the old ticket hall is being opened up leading directly from the concourse/circulating area to the Crossrail platforms. A seat is almost guaranteed as half the eastbound trains will start at Paddington.
So, changing off a fast, 125mph, train at Reading could involve a wait for anything up to 30 minutes (in which time the train one alighted from would have arrived at Paddington) in order to be able to travel in a stopping train with no toilets and a 90mph top speed. A change is necessary in either case - changing at Paddington will permit a faster journey.