Will enabling iCloud for Photos delete 174GB of local photos and sync only 25GB from iCloud?

"Will enabling iCloud for Photos delete 174GB of local photos and sync only 25GB from iCloud?:"

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Q1. "I only have 25gb of photos in iCloud, but I have 174.77gb of photos in my phone, when I try to enable iCloud storage, it says it will delete 19k photos from my phone, will it save those in iCloud?"

A1. Sync Photos through iCloud:

Enable Photos in iCloud Settings on this iPhone. If so, enable Photos in iCloud on your other Apple devices, and see if the data shows up. If your iPhone has items synced with iCloud on them, then they should pop right up at this site(it may take a while for 19K photos): https://www.icloud.com/photos

Enabling iCloud Photos on an iPhone:

1. Go to: Settings
2. Tap: Your Name (Top Item)
3. Tap: iCloud
4. Tap: Photos (so that it is green)

Q2. "My phone is full, I don’t have enough storage space to download the update. When I try to enable iCloud for photos it says it will delete the photos in my phone and sync the 25gb of photos that are currently in the cloud… So I’m scared to lose 174gb (19613 photos) Help! Thank you"

A2. Troubleshooting iPhone Storage:

Internal storage cannot be upgraded. It is what it is. So, the maximum internal storage capacity never increases, even if you upgraded your iCloud plan. Learn More here: What's the Difference Between Device Storage and iCloud Storage? Apple Support

So, what to do...

Clear Space Using my User Tips:

• How to Clear Space from your Mobile Device, and What to Clear: - User Tip
• How to Clear "Other" in your iPhone's Storage - Use Backup - User Tip

That's a lot of pictures on your phone! It is possible that turning on iCloud on your phone may wipe out your pictures. Pictures that were taken on the phone should be saved, but pictures that were transferred to your phone from a computer may not transfer to iCloud. It's a risk.

If you have  Settings>Apple ID>iCloud Backup turned on, and if you don't use iCloud, then that backup should include all your pictures. But you would have to fully reset your phone to get that backup.

If I were you, I would save those pictures to a computer or to an attached hard drive before going any further. I'might get a 256 GB flash drive like this

or this

(I've erased the brand names, because I don't know these.) You have to decide about quality and price. These cost about $30 and $50 at Amazon.

But the first rule of upgrading is, Be Sure You Have a Backup!