I will be visiting Changbaishan in the 17th of September (so very soon).

Well, that depends on which route you will take.
If you are going to reach the lake from north slope, you are not allowed to hike up, and the sightseeing bus is the only choice.

If you set off from west slope, the sightseeing bus cannot lead to the lake directly, for it will drop you at the halfway of the mountain. You have about 1400 stairs for hiking to the lake.

Dear Wendy,

Thank you for your answer.
Unfortunately, I will take the North entrance. If I can't hike up, can I go down by foot ? I saw some videos of people walking down instead of taking the bus but I can't say if it was from the north or east slope.

Thank you,
Sérima

I just went with a group to the top of Chang Bai mountain and asked this question as well. From what I heard, I am unaware of a path instead of the buses. Navigating just the buses and so forth up the mountain was complicated to say the least and I doubt I would have been able to do so without the assistance we had from locals that were hosting us doing so.

You do walk up the mountain from the buses to the Heavenly Lake.