First, let us make sure we are talking about the same thing. If you want to put the same scroll in a new, more beautiful case, then you do not make a new beracha.[1] The mitzva has not been changed significantly, as the mitzva relates to the mezuza scroll. The short break in time is not a factor when one takes the mezuza off with the expectation to return it imminently. If you take the opportunity to check the mezuza before reaffixing it (not a bad idea, as it should be checked twice in seven years), then it is a more complicated question.[2]
Regarding switching the mezuza scroll itself, we have no early sources on the matter, and so the Acharonim expend much energy looking for the closest precedent. The Rosh[3] implies that if one changes garments, each of which has tzitzit, he makes a beracha when putting on the new one. Is switching mezuzot comparable? There is a basic difference. Since one does not have an obligation of tzitzit when he is not wearing a four-cornered garment, a new obligation is created when he puts on the second garment. In contrast, the house generates a continuous obligation of mezuza, so a new beracha might not be appropriate.
According to most authorities,[4] if one removes a pair of tefillin to replace it immediately with another, he makes a new beracha. This is more similar to our case, in that the fulfillment of the same obligation continues with a different object, and we see that one makes a new beracha. However, this too may not be perfectly analogous, since many are of the opinion that there is not a continuous mitzva to wear tefillin all day. If so, putting on a new pair after removing the first is a new mitzva. In contrast, as above, there is always a mitzva to have a mezuza on one’s doorpost. Therefore, continuing the same mitzva of mezuza with a different scroll might not be a new enough fulfillment of the mitzva to warrant a new beracha. Nevertheless, the Maharam Shick[5] says that assuming one did not have the second mezuza in mind when putting on the first,[6] the earlier beracha does not cover this mezuza, and one needs a new beracha. Although some argue, the Chovat HaDar[7] follows this opinion and the Yabia Omer[8] leans toward it. (The latter suggests strengthening the need for a beracha as follows: Wait a few hours between removing the first one and affixing the other; then there will be sufficient hesech hada’at to require a beracha even according to the dissenting opinions).[9] If a previous resident affixed the first mezuza, then he who affixes one now was not involved in the beracha and should make one even if he removes and returns the same mezuza.[10]
There is another issue to consider. The gemara[11] rules that it is permitted to remove tzitzit from one garment in order to put them on another, and the Shulchan Aruch[12] says that it is forbidden if one does not plan to put the tzitzit on another garment. The issue is that of disgracing an article that was used for a mitzva by removing it from use. Since Rishonim[13] equate removing a mezuza to removing tzitzit, it is thus problematic to replace a mezuza without sufficient justification. There are significant opinions that permit it in order to replace one mitzva object with a more beautiful one.[14] However, it may be best to move the old mezuza to another place[15] that requires one.[16]