Hi ​@Sai_thitinan 

Unless you have requested a PAC code (to keep your number) or STAC code (transfer without keeping your number) from your current network then there should be no problem as you can not transfer networks without one or the other.
Your contract/connection will stay with your current supplier until a code is redeemed with the new network and the transfer has been completed. 
Simply text PAC to 65075 from the SIM connected to the phone number you want to keep from your current supplier. You will receive your code from them via SMS almost instantly, and it will be valid for 30 days. You can then use the SIM card you have bought as your number will be transferred to it.
You can then follow this guide to set up your new plan/account with iD mobile:-
https://www.idmobile.co.uk/switch
If you have already cancelled your plan with your current supplier without requesting a code then it may be worth contacting them to discuss if they are able to supply you with a PAC code so that you can transfer it across. If not you may lose the option of keeping your number.

Hope this helps x
 

Hi ​@Sai_thitinan,

I've just sent you a private message to help get this sorted.

To find it, click your profile picture in the top-right corner and select ‘Private Messages’.

You can also use this direct link to your inbox: https://community.idmobile.co.uk/inbox/overview 

We'll speak to you there.

Thanks,

Gemma M

The iD Mobile Social Media Team