Anachronistic Creatures

Characters can generally access a wider variety of damage types in Starfinder. You should make a mental note if you have any characters in your game with a limited repertoire of damage, as creatures with resistance or weaknesses to a party's primary form of damage can make an encounter too easy or too hard. Fire weakness and vulnerability are particularly commonplace and easy to trigger using Starfinder weapons, especially lasers and explosives.

Starfinder encounters are more likely to occur in environments with effects like radioactivity that most Pathfinder creatures were never designed to interact with. If your encounter takes place in space, the creatures and NPCs need armor with environmental protections or the cosmic trait to survive. If your encounter uses radiation, the creature should have it listed as an immunity, have a resistance to poison damage, or have the assumed damage and sickened conditions built into their stat block in advance so you don't need to apply it during the encounter.

Creatures inhabiting starships should have some way to survive. If intelligent, they should have a useful skill like Computers or Crafting. More feral creatures might inhabit vents or trash compactors and could have an ability like compression or a garbage spew ranged attack that helps make the creature more thematically appropriate. Likewise, creatures adapted to new worlds should have abilities that allow them to not only survive but thrive on those worlds in one or more ecological niches. A fast and easy way to approach adaptation is to look for another creature from the same world in Alien Core and skim its stat block for any abilities that help it survive the challenges of its home. For example, creatures from space usually have the cosmic trait and some resistance to cold damage, representing their ability to survive in the void.

Most intelligent creatures and NPCs in Starfinder carry tech gear. You should consider upgrading the armor and weapons wielded by such creatures in Pathfinder to their Starfinder equivalents. While the weapons could use the same statistics, applying the analog or tech trait makes it usable with the weapon grade system rather than needing compatible runes to upgrade. There are also spells and feats, such as Combat Hack, that only work if enemies have modern tech equipment. Note that while most people in the Pact Worlds carry a computer in the form of a comm unit or datapad, all tech armor has an integrated comm unit that makes carrying an additional communication device superfluous (but not unheard of). Giving a Pathfinder creature a gun is also an easy way to apply a simple but effective ranged attack if the creature didn't already have one—the image of a troll with a machine gun will stick with your players. Including tech gear in enemy stat blocks also gives the PCs ammunition (and batteries) to loot after the combat.

The integrated trait allows for equipment that players can't easily reuse in the midst of a series of encounters, although depending on the nature of the weapon and its integration with the creature, it might be salvageable with the Scavenger feat. In many cases, even the Scavenger feat isn't enough to allow a character to loot an integrated piece of gear, such as an integrated jetpack that uses a firebreathing creature's internal flames to keep it aloft.

Many Starfinder characters have access to flight and use ranged weapons that can cut an encounter with typical Pathfinder creatures short. Giving creatures additions like grafted wings or integrated jetpacks allows them to use melee tactics against flying PCs. Consider giving them other ways to close the gap, like a climb Speed or teleportation. When designing encounters, include environmental features like wind, platforms, and variable gravity.

Give most Pathfinder creatures a ranged attack. If a simple ranged attack isn't enough to deal with flying or distant foes, consider solutions that don't use extra actions such as Improved Grab, Improved Knockdown, or an accompanying hazard to that can pull the PCs closer.