The slightly quirky alternative is to take the Central Line to Lancaster Gate, walk to Paddington and then take the HEX from there. The Central Line is faster and more reliable than the Circhampolitan, generally speaking.

However, if you're not confident, then changing in Paddington is bound to be easier. The change from the Underground (Over Ground) station to the main station bridge is dead easy.

Or you can just wait for the Brenda Line to show up. Only a year and a bit until the direct services start...