After initially announcing the coming change back in May, YouTube has confirmed that it's new, abbreviated public subscriber counts will come into effect next month.
As per YouTube:
"Currently, all creators with over 1,000 subscribers see their subscriber counts displayed differently in different places across YouTube desktop and mobile apps. In some cases, the subscriber count is abbreviated (e.g., 133k) and in other places we display the full count (e.g., 133,017). To create more consistency everywhere that we publicly display subscriber counts, we’ll begin showing the abbreviated subscriber number across all public YouTube surfaces."
In practice, YouTube provides this example of how the change will impact publicly-listed subscriber counts on channels.

That makes sense from a consistency perspective, but it could also negatively impact some creators.
As noted by Marketing Land, the change could significantly reduce public subscriber listings, especially for larger accounts.
"For creators, the change means that brands may not get an accurate sense of their popularity on the platform - especially for niche influencers whose following isn’t as substantial as a channel with hundreds of millions of followers. Even for the channels with hundreds of millions of followers, if they’re at 120,450,000, their 120M subscriber count will be short more than 400,000."
That may lead to creators losing sponsorship opportunities, as they could be overlooked in favor of larger channels, despite their channel actually being much closer to parity. It's also put the onus onto the creators themselves to share their actual subscriber count - channel owners themselves will still be able to view the full, real-time count in their analytics - but some may lose opportunities outright because of the shift.
YouTube's provided this table to display how the change will impact different accounts:

Worth noting too that the changes relate to YouTube's API usage also, meaning that any app which uses YouTube subscriber counts will lose access to the full, detailed listing. That will impact analytics tools which display subscriber counts and changes over time.
Interestingly, YouTube has also noted that the change somewhat relates to creator wellbeing:
"Beyond creating more consistency, this addresses creator concerns about stress and wellbeing, specifically around tracking public subscriber counts in real-time. We hope this helps all creators focus on telling their story, and experience less pressure about the numbers."
That sounds a lot like the logic behind Instagram's experiment with hiding like counts, reducing the implied performance pressure and freeing up creators to post more of what they want, as opposed to what they think will get more engagement.
It seems like less of a factor in this case, but it may lessen the burden somewhat, which could alleviate some mental stress for creators. Maybe.
The actual impacts will be relative to each individual account, but it'll be interesting to see whether there are any significant shifts because of the update, and/or opportunities lost due to altered perception.
You can read more about the update - and see creator reactions (spoiler: most are not happy) - on the YouTube community boards.