Not a problem svasu.
I'm not sure if you'll be eligible to use the premium service for your varied application as they may consider it as complex even though it's not really the case. I suggest that you give HO a call and inquire about your options - don't be nervous
To use the premium service, you'll need to register online to book the appointment, so you might as well make the application online itself. And if you're not able to use the premium service, then you can post the supporting documents including a covering letter to inform them of your intention to vary the existing T1 application.
I find the online application the best option, it's faster and will flag up any incomplete section(s), therefore minimising the risk of making a silly mistake that could turn out to be costly to rectify.