It is important to check that you've got enough blank pages in your passport when you start the visa application process. Majority of embassies require you to have 2 blank pages per visa. If you don't have enough blank pages, you would need to renew your passport.
If you are renewing your passport, it'll need to be done prior to starting the visa application process. Once the visas and letters of invitation are issued they are tied to the passport. So if you change or loose your passport, all of the visas and letters of invitation you got so far will no longer be valid.
It is especially important to remember it when applying for a Turkmenistan transit LOI (letter of invitation). Once the application for it is submitted, no changes will be allowed, and you will only be able to enter Turkmenistan with the passport which was used to apply for the LOI.
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