Hi Venkatesh,
If you are still within the first 30 days of your Azure Free Trial you will still be able to use your $200 credit even if you switched over to Pay-As-You-Go.
After the first 30 days of Azure Free Trial you are required to switch to Pay-As-You-Go or else your subscription will be disabled. Once you switch to Pay-As-You-Go after the first 30 days you still have access to the free services for the remainder of the 12 months (11 months). As long as you stay within the free limits there is no charge.
On page below, click See all free services to see list of free services:
https://azure.microsoft.com/en-us/pricing/free-services
Exactly what is included for free can sometimes be confusing, so if you have a question about a specific service related to what exactly is included free please ask.
In regards to switching your subscription from pay-as-you-go back to the original state it was in, assuming you are still within the first 30 days, I don't believe it is possible. You are welcome to create a billing support request and ask them (billing support is free). Instructions below:
Use this link to start the process:
https://portal.azure.com/#view/Microsoft_Azure_CostManagement/Menu/~/support/openedBy/AzurePortal
Fill out Summary, Problem type, Problem subtype, click Next. There will be a delay, then some recommended solutions will be shown. Do not click on any of the recommended options shown.
In the upper left corner, click Return to support request
Click Next to move to Additional details tab, fill out required information, email/phone, etc., click Next, review details and submit case. After submitting you should receive an automated email with a few minutes and a support engineer should contact you within 1 business day.
Please click Accept Answer and upvote if the above was helpful.
Thanks.
-TP