I want to change my apple ID but need time to swith over all my contacts before I change it. Could I get an allias address, switch my contacts and then make the allias address my new apple ID?

If you create/use a new Apple ID, you will lose access to some of your data and the purchases made with your existing Apple ID. Have you considered doing a name change your current ID instead?

Note the restrictions in the article.

Apple ID Name Change.        

 What to do after you change your Apple ID or password - Apple Support    I suggest doing this before you change your Apple ID or password.

You can do it, but it may not be a good idea for you. Most don't do this unless sharing an Apple ID on, say, two devices. See How to create a new Apple ID https://support.apple.com/en-us/HT204316. See the data implications below.

Some of these things may apply even though not changing an Apple ID: What to do after you change your Apple ID or password https://support.apple.com/en-us/HT204071

 Keep in mind that some things available under a first Apple ID will not be available under a new Apple ID. For example, you won’t be able to transfer anything purchased from the App and iTunes Stores to the new ID. And, there are other issues to be addressed.

See How to switch Apple IDs on your iPhone or iPad

https://www.imore.com/how-change-apple-id-your-iphone-or-ipad

And see what Eric Root says on this matter here  https://discussions.apple.com/thread/251483565and

what Winston Churchill says about it here: https://discussions.apple.com/thread/7452747

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You also can set up family sharing between the IDs to share purchases made with them. See:

Family Sharing  https://support.apple.com/en-us/HT201060

Set up Family Sharing https://support.apple.com/en-us/HT201088