Can I use a magic item while it is being upgraded?

[wraithstrike]

Upgrading a belt or head band from a +4 to a +6 would take 20 days. Since the magic item creation rules are designed to be easy on the player it makes no sense to have them go without the item for almost 3 weeks.

Another example is taking a sword from a +2 to a +3. That is 10 days. If the item was not expected to be used there would need to be rules that said something like "the sword is only a masterwork sword while it is being upgraded" or something similar that denotes penalization.

Nothing is mentioned that actually limits use of the item in the rules. Any GM who does this is making a houserule.

[Azothath]

The simple assumption is that upgrading is crafting.
I believe the crafting rules require the wizard(or crafter) who is doing the enchanting to be with the item 8hrs a day without interruption or distraction. Yes there are rules for temporarily halting the process and those would have to be used.
In PFS it's a moot point as this all happens between adventures.

[_Ozy_]

Azothath wrote:

The simple assumption is that upgrading is crafting.

I believe the crafting rules require the wizard(or crafter) who is doing the enchanting to be with the item 8hrs a day without interruption or distraction.

Nope. A crafter can work with an item one day, let it be used the next, work on the 3rd day, let it be used on the 4th, and so on until the enchanting is finished.

The only caveat is that you need an interrupted 4 hour block of time to make full progress (2 hours if you boost the DC by 5). If you don't have uninterrupted time, the rules assume you're spending it during breaks during the day, you make 1/2 progress:

Quote:

If the caster is out adventuring, he can devote 4 hours each day to item creation, although he nets only 2 hours’ worth of work. This time is not spent in one continuous period, but rather during lunch, morning preparation, and during watches at night.

[Azothath]

the quoted text in more detail is link;

PRD:Magic Item Creation, CRB wrote:

The creator also needs a fairly quiet, comfortable, and well-lit place in which to work. Any place suitable for preparing spells is suitable for making items. Creating an item requires 8 hours of work per 1,000 gp in the item's base price (or fraction thereof), with a minimum of at least 8 hours. Potions and scrolls are an exception to this rule; they can take as little as 2 hours to create (if their base price is 250 gp or less). Scrolls and potions whose base price is more than 250 gp, but less than 1,000 gp, take 8 hours to create, just like any other magic item. The character must spend the gold at the beginning of the construction process. Regardless of the time needed for construction, a caster can create no more than one magic item per day. This process can be accelerated to 4 hours of work per 1,000 gp in the item's base price (or fraction thereof) by increasing the DC to create the item by +5.

The caster can work for up to 8 hours each day. He cannot rush the process by working longer each day, but the days need not be consecutive, and the caster can use the rest of his time as he sees fit. If the caster is out adventuring, he can devote 4 hours each day to item creation, although he nets only 2 hours' worth of work. This time is not spent in one continuous period, but rather during lunch, morning preparation, and during watches at night. If time is dedicated to creation, it must be spent in uninterrupted 4-hour blocks. This work is generally done in a controlled environment, where distractions are at a minimum, such as a laboratory or shrine. Work that is performed in a distracting or dangerous environment nets only half the amount of progress (just as with the adventuring caster).

A character can work on only one item at a time. If a character starts work on a new item, all materials used on the under-construction item are wasted.

Ultimate Campaign also talks about this topic but covering variances on the CRB such as Cooperative Crafting, Upgrading & Altering, crafting as a business. The Downtime system is a feature of Ult Cmpgn.

The CRB quote above answers the original poster's question. You should point that out the relevant text to your GM. It's also a change to the 3.0/3.5 DnD rules in case your GM knows those well.
House rules are okay and every GM will have their own. As this is a Rules board generally answers will only go as far as RAW.