From your question I understand that you want to change over your milk cutlery so you can use it for meat, or vice versa, to change your meat cutlery and use it for milk. This is a good question that I will get to in a moment. But first I want to point out that to “toivel” dishes does not kosher them – not from milk to meat, or from non-kosher to kosher. To “toivel” dishes means to dip them in a mikvah (or the sea etc). This is required for all metal or glass vessels that were made or purchased from a non-Jew. The laws are various and need to be studied, but suffice it to say that dipping the vessels in the mikvah, though required in order to use the dishes etc, cannot remove any tastes already absorbed into the vessel. For that you need to do “hag’alah” – which means koshering the cutlery by boiling it up in a pot of boiling water. The cutlery (at least 24 hours after last use) must be put into a pot (which has not been used for 24 hours) and then brought to a strong boil. [There are other ways to kosher dishes, but for metal cutlery, boiling them is the simplest].
So, to answer your question – may one boil up their cutlery to change it from meat to milk or vice versa? One may certainly kosher cutlery from non-kosher to kosher, or from kosher to Passover use. However, some authorities question the practice of changing vessels from meat to milk, or back, as they are afraid that people will come to rely on having only a few vessels and be constantly swapping them back and forth from milk to meat by koshering them, and in the end there will certainly be a mess up and they will be used for the wrong food (see Magen Avraham, 609,11). Though there exists some argument about this practice (see the Aruch HaShulchan 609, who completely rejects it), we generally try to be strict in this question and follow this opinion. However, in many cases everyone agrees that one can kosher things from milk to meat or vice versa – for example
1) if they became non-kosher, or needed to be koshered for Passover, one can use the opportunity to change their status between milk and meat.
2) if a long time has elapsed since their last use (such as a year).
3) in a time of great need, for example if one is totally lacking the needed utensils.
4) if one received them as a gift and wants to change their status.
Therefore, if you want to change the cutlery over for a good reason, you may certainly go ahead and do “hag’alah” to kosher them, and then use them for either milk or meat. For example, if you have decided to become a vegetarian and now want to have an only dairy kitchen. Or you inherited your grandmother’s kitchenware which was dairy, but you would rather have more meat dishes. But if you want to have only one set of cutlery and be constantly koshering it so as to use it for milk and meat, it would be correct to refrain from such a practice.
In any event, before you go ahead and kosher anything via “hag’alah”, you should speak to someone who is knowledgeable about the details of the process, or be in touch with our site again to learn the laws of koshering.