First off, your PageRank, the green toolbar stripe will not increase if you tweak META tags. Secondly, your pages' ranking in the SERPs are highly unlikely to change just because of META tag tweaks. Especially in the good search engines. Just make sure you have unique titles on every page and perhaps a good description, since that might actually show as the snippet in the search results.
Changing meta parts will work for you, for this or next update only. Remain fix with same meta part for all the prime pages at least for 01 year, judge keywords performance and PR, then only decide to change meta parts so frequent. First finalize your points with the selected keywords with which you want to be optimized.