How to substitute a character with a diacritic only when it's preceded by an another character?

[Fyteria]

I'm making a font for my conscript. I've made some ligatures for syllables. Vowels can be represented either by symbols or diacritics:

FontCreator_FtcbP7jfPY.png (6.17 KiB) Viewed 259 times

Symbol and diacritic on the left is for vowel a, while on the right is for vowel e.
I want to make a substitution rule, where the vowel symbols would turn into diacritics only when they're preceded by a syllable (e.g. pa (syllable) + a (symbol) = paa (syllable + diacritic). I tried doing this with single and multiple substitutions, but it affects even isolated and initial states of these symbols, I don't want that.

FontCreator_JmyYlv8S3w.png (12.86 KiB) Viewed 259 times

FontCreator_ElZlPIVtSt.png (12.14 KiB) Viewed 259 times

Such symbols should turn into diacritics only once and only if they're preceded by a syllable.

FontCreator_nPo9L6CmSk.png (21.91 KiB) Viewed 217 times

FontCreator_PKXdW8rlJ5.png (3.63 KiB) Viewed 259 times

Currently, it looks like this (screenshot above).

[Erwin Denissen]

You will need a contextual lookup. See the Swashes for an example:

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