When i upload a file lets says as standard IA, it is successfully uploaded and when i try to change to store class it shows a summary of all storage class which has column minimum storage time (30 days)?

Does that mean that the object which is just uploaded should be stored for 30 days then only we can change its storage class? if yes then why i am able to change its storage class instantly?

Even on Docs there is no mention

To quote from the S3 Pricing Page (the fineprint, emphasis mine):

S3 Intelligent-Tiering, S3 Standard-IA, and S3 One Zone-IA storage are charged for a minimum storage duration of 30 days, and objects deleted before 30 days incur a pro-rated charge equal to the storage charge for the remaining days. Objects that are deleted, overwritten, or transitioned to a different storage class before 30 days will incur the normal storage usage charge plus a pro-rated request charge for the remainder of the 30-day minimum. This includes objects that are deleted as a result of file operations performed by File Gateway. Objects stored for 30 days or longer will not incur a 30-day minimum request charge.

So you can change the storage class at any time, but you’ll still pay for at least 30 days.