With everything in place, he wanted to test this out. He knew his little sister was asleep, so he decided to test it on her. He didn't plan to permanently stay there, just wanted to see if it worked.
He headed out, but passed her room. With the settings, the chronivac had warned him, so he decided to take the warning. He went to a storage closet and grabbed out a pair of her old diapers, before heading back out.
With the diaper in hand, he headed over to her room. The lights were out, so he couldn't see much. He had the bed location memorized, however, so he went towards it.
Stripping his clothes off, he hid them under her bed before pulling the sheets out, uncovering her legs and - more importantly - her crotch. He slipped under her clothes, and crawled into the vulva, as stated by the Chronivac.
As he entered, he felt the changes take effect. He shrunk down as he regressed to six years old, the same age as Mary. He continued to crawl in, and found quite a bit of space in there. He could tell he was shrunken beyond his usual height at that age, as he fit in perfectly.
Getting into position, he could feel the Chronivac do the proper changes. He felt a shift in his nuts, as they were repurposed into Mary's womb, albeit still retaining their original purpose alongside it. With the initial modifications done, he realized this was the time.
With the limited movement he had, he managed to slip on the old diaper, fitting a bit tight but not the worst. The pattern would be embarrassing had he been seen in it, but he didn't dwell on it, since only he would see it. He was surprised at how comfortable it was, with the walls being soft yet also contracting slightly.
He didn't have anything else he could do from here, and there wasn't anything he could see. He finds the sensation odd, but not the worst; "I could handle being in here for a while!"
He only went in for a test though, and as he had just confirmed, everything worked out great. Now he had a choice to make...