Question moving from Magento 1.x to 2.x - what to do with extensions

1. Make a list of extension that implemented,
2. It is better to filter which extensions that you gonna need or you don't need it anymore after the upgrade.
3. Find the source of every extension that you want to implement (whether you buy it or develop on yourself or third party vendor)
4. If the extension you buy and modify from extension developer company like amasty, magecomp, fme, mageplaza, etc. Then, it may be the best that you contact them first and check is there any version fit into 2.x, you may do less recode if you purchase the support.
5. If the extension develop on yourself, yes you may do want to check the change log and doing a recode.
6. If someone else developed specialized extension for you, then you might need to contact them to do a less recode.

I've been doing this from 2.2.x to 2.3.x,but if you want to find any third party to doing it for you, there are lot of them. Some that I've been / will be working in near future with : https://www.emizentech.com/ , https://www.limecommerce.com/ , https://amasty.com/magento-services.html , https://magecomp.com/magento-services.html , and https://www.fmeextensions.com/magento-services.html