Disclaimer: I have only one existing a/c with CIMB (i.e. FastSaver), and not familiar with StarSaver. I am not an expert. I assume you have already read the T&Cs for Existing Customers and understand the key points.
T&Cs for Existing Customers:
https://www.cimb.com.sg/content/dam...overning-casa-bonus-interest-promo-23-etb.pdf (must read)
Below are just some additional comments for your situation:
Since you have a high balance $100k for FastSaver and a low balance $1k for StarSaver as at 31July, read T&Cs for Existing Customers – point#3 ‘Qualifying Accounts’ and point#4 ‘to qualify for this promotion’. After reading, it is better to contact CIMB to confirm if you can use existing StarSaver a/c for this 3.5% promo and only the $1k balance (not the total of $101k) as at 31July will be used for comparison in the calculation of bonus interest. If you do not know, you should also confirm with them if StarSaver is classified as Saving a/c (min.$1k fresh fund) or Current a/c (min.$5k fresh fund)
For Existing customer, it is important to register for the promo, and after registration then transfer in the minimum fresh funds of $1k/$5 before 31Aug to ‘lock’ into the promo. You should ideally use another bank a/c (not that eSaver a/c -Std Chartered) to do the transfer. Read T&Cs point#5 (‘fresh funds’). The promo give 6 interest-earning months of Aug2023 to Jan2024. Although you may not be able to maximise for the month of Aug2023, the remaining 5 months is still a good option to play with regardless you actually decide to transfer your main funds into StarSaver after Aug.
I assume you are currently able to earn equivalent bonus interest with eSaver a/c (Std Chartered). Read point#1 and point#5 of T&Cs – it appear to be safer to transfer your main funds (‘fresh funds’) into StarSaver from 1st Sep rather than transfer within the same month of Aug where you have transferred $$ out from FastSaver on 1 Aug. In addition, you can also wait until 20th Sep to verify bonus interest earned for the $1k/$5k fresh deposit, since prevailing interest will be credited on 31Aug and bonus interest will be credited on 20Sep.