I think Henk and i are pretty much in agreement, but i will throw down my 2 cents on the matter anyway...
It is part of the culture of the keris to regularly clean and re-stain the blades (though staining is not a common tradition on the Peninsula as i understand it). It is a way of doing honor to the blade and to maintain the spirit and ancestral energy which is supposedly held within.
Also, as Henk already pointed out, dress was routinely changed as wooded parts wore out or social status was upgraded. Of course this is all as seen from the perspective from
within the culture.
Now many of us are collectors of keris from
outside the culture. Still, i see nothing wrong with attempting to maintain my keris from a somewhat indigenous perspective. So if i receive a keris that is in a bad state of condition i may well choose to renovate that keris to bring it up to respectable and "original" shape. I would therefore clean the blade with pineapple juice and stain it with warangan (a mixture of lime and arsenic) to revitalize the blade. If the dress is particularly damaged i may attempt to repair it as best i can.
Sometimes you will find a keris that is already a mishmosh of various styles that has been assembled by some dealer somewhere just for the sake of sale. Under those circumstances i might choose to replace a hilt or other part to return the keris to what i perceive a a more correct cultural dress for that particular keris.
What i do
not generally choose to do, however, is to upgrade the dress of my keris just for the sake of presenting a prettier ensemble. Some collector do this, but i find it counter to my own intentions of collection. I like to collect keris with a "history" and much prefer to find a keris that is as close to "original" condition as possible.
But if you are wondering whether cleaning the blade of your keris will hurt the value of that keris i would have to say no, it would not, unless of course you did some kind of irreversible damage to the blade. If you do a search on this forum for keris cleaning you should come up with some fairly clear instructions on how to safely clean your blade and prevent further rusting. Staining with warangan is a bit trickier and you might want to leave that to a professional or not stain it at all, but nobody likes a rusty keris blade so i would recommend that if there is active rust on your blade you take care of it and then oil the blade regularly.