Not sure what kind of change you are seeing. A sudden, 'poofy' textured coat usually indicates illness. Of course diet is very important and a change in nutrition can result in a change in fur texture - more glossy, full fur if nutrition and health are optimal and dry, brittle, thin fur if nutrition or health are poor. There are many specifics within the diet that can change the fur of your pet too.
My megacolon rats would rarely ever get the oxbow regal rat that the rest of the rat family lived on. They ate almost exclusively baby food. After a while, I began experimenting more with their diet, and they are eating much more of the oxbow kibbles and less baby food. I noticed the girl's fur has become a little coarser (she used to be baby soft) and the boy's is a bit coarser too and he's lost a little weight. The girl on the other hand is as chubby as ever, maybe moreso.
I was wondering about this too. I think the meat based baby foods had more animal fat in them, also more water, so that might have been why their fur was softer. I wanted them to eat more oxbow because I was worried about nutrition and vitamin deficiancy after eating JUST baby food. I think I've found a pretty good balance for them, but I wonder why exacly the girl in particular has slightly rougher fur now.
It can show diet issues, but it doesn't tend to be sudden onset. A soft shiny coast without too long hairs is very much a sign ththey have everything or more that they need. Drier fur can feel coarser and is linked with oil content. Older fur can be down to a lack of protien delaying moult and can give coarser longer coat. There's more stuff as well but it depends on what your girl is like
Grizz has a shiny, sleek coat and has the entire time I've had her. I think Lee has finally grown her adult coat, so she's coarser than she was because of that. Yesterday when I was playing with her I noticed that the fur on her back was coarser than I've ever felt-- it was normal on her belly and head. They're both on Oxbow and I am giving Lee additional protein although I think I'll be stopping that soon as she's approaching four months. I am just concerned that she's getting too much/not enough protein.
I know she's drinking enough water and eating the Oxbow as well as the veggie/fruit mix every day. The only difference between the girls is the additional protein-- that, and Grizz eats more.
Different rats dip often have different requirements, they like to be awkward. To check the protien have a look at her real as well as coast. If it's thick and round she had pleanty (possibly a bit much if anything) if it has light edges visible shes fairly normal for a youngster, if it's quite Square edged she's short of protien at the moment. Usually that means she having a growth spurt.
They grow into there adult coat at 5 to 6 weeks old, after that a does coat is fully mature