Changing country - Apple ID

I posted general information below. I am unsure about the specific things you mentioned. Use the support links I gave.

See Change your Apple ID country or region https://support.apple.com/en-us/HT201389 See this heading especially What to do before you change your country or region

Use this link in the document if you need to View your Apple ID balance https://support.apple.com/en-us/HT202359

Contact Apple Support If you can't complete any of the steps, can't change your country or region, or you have less store credit than the cost of a single item, contact Apple Support. Note that you could just purchase something of value greater than or equal to the amount of your store credit or ask  iTunes Support to zero out your account balance using this  iTunes Support Form https://support.apple.com/en-euro/itunes-contact

And you also can contact iTunes worldwide support via text or chat using this link https://support.apple.com/choose-country-region/itunes Choose your country. At the bottom of your screen use Tell us how we can help to select among support options. You should be able to get telephone or chat support.

You shouldn't lose your paid apps or your purchased movies. If the version of the app in the UK store is different than the version in Poland's store, you may not be able to update your apps.

I moved to the USA

Did you read donv's post?