Just read the two posts you suggest...It only tells me what I knew...16GB max RAM supported, and OP already installed 4GB RAM to the existing 4GB so it totals 8GB right now. Nothing suggests it is not possible to remove and replace the two 4GB RAM and replace them with two 8GB units.
As for lost 'investment', adding 4GB RAM only seems to be about $15-20 per unit (per web search immediate hit, w/no serious digging for lowest price available!)...hardly a huge loss of money spent. Two 8GB is about $30-35 per unit, so about $70 to max out RAM. OP noticed when running multiple programs the improvement, even in going to 8GB total, so "why not go to 16GB to try to get more performance?", is the way I look at it.
OTOH, there just might be some hardware issue that is slowing performance, and throwing more money at the issue with the exiting PC might not resolve the speed issue. Right now I have encountered a speed limiting issue with my laptop, and reloading OS did nothing to improve it, even with few applications to burden it! So rather than going thru the effort to change to SSD, it might be better to cut future losses and get a new PC, and put the new SSD in an external enclosure and use it to have data redundancy via its usage.