Say it is 23C at bedtime but it is expected to get to 18 or 19C later, do you put your child to sleep in the 2.5TOG which is for 16-20C or put in 1.0TOG then change to 2.5TOG later in the night (and risk waking up baby?)

It can be difficult to work out in advance what may be the most appropriate clothing/bedding for baby when sleeping.
Red Nose recommends dressing baby according to the room temperature, rather than trying to modify the room temperature with heating or cooling.

I wish I could give you some specific guidance for dressing your baby, but every baby is different.
Red Nose has the following general information.

Dress baby and use layers as you would dress or use layers yourself: to be comfortable, neither too hot nor too cold.

Dress baby for sleep and add/remove lightweight blankets to ensure baby’s back or tummy feels comfortably warm to the touch.
Remove hats, bonnets, beanies and hooded clothing from baby’s head as soon as baby is indoors.  - Always sleep baby with head & face uncovered.

A good way to check baby’s temperature is to feel baby’s back or tummy (don’t worry if baby’s hands and feet feel cool - this is normal).
Whether the baby has a cold or infection or another special need.
Consider how many layers that you as the baby’s carer are wearing comfortably.
https://rednose.org.au/article/bedding-amount-recommended-for-safe-sleep

As long as baby is put down to sleep on their back, and that baby is dressed appropriately for the room temperature – not overdressed or under dressed – with their head and face uncovered, you can feel reassured that you are protecting baby from overheating.

We don’t believe that it’s necessary to use a room temperature monitor, or to leave the heating or cooling on all night, as long as baby is dressed appropriately for the temperature of the room – not too hot, not too cold.

https://rednose.org.au/article/how-to-dress-baby-for-sleep

TOG ratings are mostly an approximate guide & are usually based on European climate.
If your baby is cold, they will usually wake up - Remember - is my baby hot/cold/wet/hungry…....etc?