Did you add tempered glass &or screen protector.
These may create problem on resolution.
When i bought A8, the resolution is quite bad.
At the store, handset do not have this resolution issue as there is no tempered glass
Then i decide to remove the tempered glass and screen resolution is much better.
For 1 on 1 exchange, it is usually within 1 week, and depend on the Manufacturer Service Center.
You need to bring along the receipt, as it based on the date of purchase.
There was a user using iPhone with similar problem, but the Service Center refused 1 on 1 exchange, within the purchased date.
Giving 3 options, but replace is not new set, repair, etc.
I did had some experience with Samsung Service center.
On 1 case, the screen do have scratches (i damaged it), and was replace the screen Free, without question.
So, in your case it depend, either they give you 1 on 1 exchange or replace the screen as warranty issue.
I would say, that they would just replace the screen, if it is the problem.
As for the heat issue, this is common problems on all phones.
For heat issue, (1) take a break and the phone heat will go away very fast; (2) do not use and charge at the same time, i normally did a fast charge before leaving home. Fast charge will charge to 49% to 50% within half hour.; (3) go to recent tray (left buttom below) and clear the background apps, include updating apps.
(4) Use the phone manual to check for where are several antenna location. That is when you hand or casing with thick back cover, block the antenna, will generate heat, as more battery power are required.
Most Manufacturer Service center will give 1 on 1 exchange if it is due to manafucturer problem.
Such as Xiaomi phone, as i told the user to visit the only 1 Service center.
The user feedback to me that they did 1 on 1 exchange as it is manufacturer defect.
Starhub is not involve on this matter.
I am not Starhub staff &or related to.