how do I get my old pictures with a new icloud

I believe what you need to do is to:

1. Copy all of your photos and videos out of the iCloud Photo Library associated with your old Apple ID, to some place that ISN'T associated with that Apple ID.
2. Import the photos and videos from that place to an iCloud Photo Library associated with your new Apple ID.

For instance, you could use the iCloud Web interface to download a bunch of photos to an area outside of your iCloud Photo Library (step #1), then log out, log back in again with the new Apple ID, and upload the photos to the new iCloud Photo Library (step #2). (Note that when you download photos this way, you have to choose between downloading originals, and downloading modified copies.)

https://www.icloud.com/

iCloud is intricately linked to an Apple ID. Should you decide to change your Apple ID, it's important to note that the data stored on iCloud won't be directly accessible with the new Apple ID. This means that if you switch to a different Apple ID, you might encounter difficulty accessing or retrieving the information previously associated with the original Apple ID within iCloud.

This association between iCloud and an Apple ID serves as a unique identifier and authentication mechanism. It helps maintain the privacy and security of your data while ensuring that only authorized users, linked to a specific Apple ID, can access and manage the information stored in their respective iCloud accounts.

In essence, when you change your Apple ID, the connection to the existing iCloud data becomes detached, resulting in the potential inability to view or retrieve the previously stored content using the new credentials. Therefore, it's essential to carefully consider the implications and potential limitations before making changes to your Apple ID to ensure a smooth transition without any loss of data access.

Your data is not lost, it remains intact with the old Apple ID however you may have no access to it until you use the old Apple ID on a device.