Regan's first appearance is while she sleeps in her room. Here, it's difficult to see her face, but she looks soft and young. Her face is unmarred. Instead, she looks very peaceful. This look remains for her first four appearances. Her hair seems to be crimped or curled and then brushed out, and she has lightly wavy bangs. Her lips are a natural pink and her cheeks are rosy. Her whole appearance emphasizes her youth and she looks as though she hasn't even hit puberty or lost her baby fat. Her face is dewy, and the makeup is extremely light - you can even see the tip of her nose shine!
There's then a section of appearances where her makeup changes are either extremely subtle or just a product of the lighting. She seems to look paler in the night scene and the fifth day scene at her house. She looks a little more sunken, but again, it looks so natural that I'm pretty sure this is just the lighting.
During Regan's first surgery, there is a subtle, but clear change in her makeup. Her cheeks are seemingly blushed grey instead of pink, and her skin looks less clear and more splotchy, as though she has some bruises forming. The room is cool-toned, which is probably aiding the effect. She turns paler in the seventh scene at her house, in which her eyes roll up (probably white contacts that cover her pupil, maybe sclera contacts?) and she develops a scratch or slap on her cheek.
In Regan's second scene at the hospital, she's drenched in sweat. At the hypnotist's office, meanwhile, she is dry, but her face is extremely pale and the skin around her eyes is red and wet. Her lips look paler, but not in a way that looks cracked quite yet.
The third hospital scene is the most clear progression towards her eventual transformation. Her lips have been foundationed over to produce a cracked or chapped effect, and her face has several scratches on it. The injury makeup continues in her appearance at home, in which she stabs her vagina with a crucifix multiple times. Here, she's scratched up, she's covered in blood, her lips are still cracked, there's a nondescript injury at the crotch area (she's clothed, so all that's shown is blood on her crotch), and there are strange wrinkles at her eyebrows, which I don't think the actress can produce naturally. They look vaguely canine in the way they bulge on her face.
By the time of Father Karras's first visit, Regan again changes dramatically. Here, her lips are completely colorless, her entire body (but especially her face) is covered in bruises and raised cuts, her face is totally unblushed, and there are bandages on both of her wrists. The actress is wearing a pair of greenish-yellow color contacts here. At one point, Regan pukes some kind of pea soup colored liquid, which stains her pajamas and covers her chin. During Karras's second visit, she looks much the same, but we're given a good look at her teeth, which are stained and disgusting. During Karras's third visit, she is shown drenched in sweat, her hair greasy and tangled. Her stomach is shown, on which there are numerous bruises and her ribs are showing. "HELP ME" is slowly written in raised skin from inside her body, which was likely done using a skin dummy instead of the actress's actual stomach.
Before her exorcism, her face is shown a few times as she waits for Father Merrin. Her face is now wrinkled to look like the Pazuzu statue, which has a noticeable canine appearance. Her undereye is red and wrinkled, and her pupils are noticeably tiny.
During the first exorcism, Regan goes through several makeup changes. In various moments, Regan spits more green puke (leaving her with it crusted on her chin and upper lip for a period), her eyes go completely white again, her tongue is shown (now purple and pointed), her head turns 360 degrees, her face melds with Pazuzu's for a moment in what is likely special effects, and she once again appears drenched in sweat with her hair plastered to her head (though this goes away a scene later). Nearly her entire body is covered in cuts, excluding her neck, and new ones appear on her legs as holy water is thrown on her. Her undereye is extremely bruised, and seem to grow redder throughout the scene. Her hair sits in greasy curls on her shoulders. Her eyebrows might be covered, but I'm not sure. This overall makeup transformation stays mostly the same until the final scene.
In the final scene, Pazuzu has been driven out of Regan and some time has passed since the exorcism. Her makeup is now closer to that of her first few scenes, but noticeably, her face is still bruised and her cuts have become scars. She doesn't look exactly the same as she did before, but she's happy and healthy.