Started 20mg lexapro, (escitalopram) was on 40mg prozac for about 8 months. i switched straight over. since prozac has long hl, is serotonin syndrome possible?

Practically NO: Seratonin syndrome is real but extremely rare. Only about 2 per million will,experience it. Those that I have seen are on many agents that elevate serotonin, or interfere with its metabolism, allowing it to accumulate. St Joh's Wart is a particularly bad agent, causing this when taken along with SSRIs like you mentioned. So, while it is possible for you to get it, the odds are against this.