Apple is reportedly planning to hold an "enormous" iPhone upgrade event this week in its stores, in an effort to boost sales of its latest smartphones.

The company will be sending out emails to iPhone users who are eligible for an upgrade to notify them of the event and invite them to their local Apple Store to upgrade to a new iPhone 5S or iPhone 5C. It's worth pointing out that this is not an iPhone launch event, as the next-generation iPhone 6 is not due to hit retail for a few months; it's just an effort to boost sales by inviting users to upgrade to the latest iPhone models.

At the same time, this event is separate from Apple's trade-in program. The news comes from 9to5Mac, who claims to have learned about this upcoming event from a "source with knowledge of the initiative."

"Beginning May 8^th, Apple will be contacting upgrade-eligible iPhone users with older iPhone models via email to come into their local Apple Store to update to a new iPhone 5s or iPhone 5c," reports the publication.

"This program blends in nicely with Apple's Apple Store-based iPhone hardware trade-in program, which allows customers to turn-in older iPhone models to purchase a new model at a discounted price. Apple, in recent weeks, has increased the buyback prices for iPhones in its trade-in program, according to a source," 9to5Mac further adds.

According to the publication, Apple will be holding this iPhone upgrade event in multiple states across the U.S., and it expects great traffic with numerous interested customers, along the same lines as for new product launches.

Moreover, Apple is reportedly preparing several other new marketing and sales initiatives, also in a bid to increase sales of iPhones in its retail stores across the U.S. Aside from the annual iPhone launches, the upcoming upgrade event this week will be one of the company's most notable iPhone sales events.

Lastly, such sales strategies are also likely aiming to boost sales ahead of a big product transition, as Apple is widely expected to launch at least one large-screen iPhone 6 model this year.