GA-20 to GA-40 switch: MSers may switch between GA preparations as a result of local skin reactions. GA-40 is only administered 3x per week and has been shown to be better tolerated than the daily GA-20 formulation.
Lack of efficacy: I would not recommend switching between GA preparations because of lack of efficacy or perceived lack of efficacy. If you have had a suboptimal response to one GA preparation it would make sense to switch classes. In general, I tend to escalate treatment rather than switch to another moderate efficacy DMT.
Other DMTs: Provided the baseline screening bloods are fine and there are no specific contraindications I see no reason why GA can't be used after any of the other licensed DMTs. However, if you are switching due to a suboptimal response I would recommend a more efficacious DMT. There is reasonable real-life data that shows switching upwards (escalation) gives a better overall response rate than switching to a similar efficacy DMT (horizontal switching). I suspect that most MSers who switch to GA will be doing so for family planning reasons, because GA probably has the best safety record in pregnancy. Another reason to switch may be an adverse event on another DMT, for example a persistent lymphopaenia.
Special circumstances: The presence of other specific comorbidities may make it difficult to switch from certain DMTs to GA. These could include generalised urticaria or a known allergic reaction to GA in the past.