I have had a LOT of questions in store about this - I made a ruling and it was accepted but then of course someone watched a tournament somewhere and it did the opposite so the question is raised again.

Account Siphon: Make a run on HQ. If successful, instead of accessing cards you may force the Corp to lose up to 5, then you gain 2 for each credit lost and take 2 tags.

FAQ as of 26 September on replacement effects: "An ability that uses the word "instead" is a replacement effect. Once an effect has been replaced with a new one, no other effects can be triggered off the original effect, regardless if they are regular effects or additional replacement effects"

That is open and closed isn't it? Account Siphon is a replacement effect so a runner cannot make the successful run and then access cards instead. They may choose to siphon coins and take the tags if they want but they can't then go "No, I'll access cards instead."

Please correct me if I am wrong. I am aware of the FAQ clarification of "The Runner can choose to not use the ability on Account Siphon

If successful trigger is at step 4.4 of a run. Access is at 4.5.

The runner doesn't have to make a decision until step 4.5.

This is good as you can choose to access after the corp has dumped credits rezzing ice, assets, and upgrades or dumping them to a trace.