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The current "free-to-play" model for Project Spark will soon shift to a totally free model when the game creating game moves to the free Incubation Engine on October 5. This date will mark the release of the "biggest update and platform change" thus far for Project Spark.
What does that mean? This will automatically unlock previously paid downloadable content for new and existing Project Spark users. Microsoft will pivot from producing DLC and active feature development to encouraging more user generated content and opening the Project Spark experience.

Project Spark’s goal has always been to empower creativity. We’ve been an incubation engine for ideas from epic to artistic and we plan to continue doing so. Project Spark inspires and empowers over 200,000 creators who have shared tens of millions of custom objects, behaviors and experiences. Every day we see anywhere between 300 and 400 new games being uploaded on our platform. Our support of these creators and our communications with them will continue in an open, free and collaborative environment.

In light of these changes to a fully free model, all players who have purchased Project Spark digital content on or after July 28, 2015, purchased a retail disc and activated it by product key redemption on or after July 28, 2015 or have a remainder balance of purchased in-game tokens will be entitled to Microsoft Store credit equivalent to their money spent. Microsoft Store credits will be awarded automatically to all users within 30-60 days after October 5. If you have additional concerns, we have a full FAQ on our refund process.

On top of this, the content packs that were released today will be the last planned DLC releases for the game. This includes 46 asset packs with over 2,000 assets released in the previous few months. They make sure to note that this also includes an end to the episodic adventures, including those for Conker's Big Reunion, saying that it's now up to the community to "continuing his story in whatever ways you want." That is to say, Microsoft has once again abandoned Conker.

Here is a bit more on some of the major changes coming in the October 5 update.
• The maximum terrain limit in creations has been doubled (on top of Massive World Builder)
• The maximum prop limit in creations has been increased by 500 (on top of Massive World Builder)
• 200+ new assets will be added in game, including a dragon, goblin warlord, bog biome and a set of new primitive building blocks
• Each creator will now have a total of 100 Upload Slots for their creations
• UGC ranking will be based on downloads and favorites, instead of upvote/downvote ratings
• The Project Spark Marketplace has been removed
• The Project Spark Tour and Treasure Trove have been removed
• Achievements have been revamped for players and creators

(via Project Spark)