I have no problem doing spinal after epidural provided that the epidural hasn't been bolused in the last hour or so. If the epidural has been bolused, especially heavily, there are a couple things to keep in mind. The epidural space can get dilated, or expanded, and compressing the dura and subarachnoid space. this has a couple of issues, some of which are you can get false positive with spinal placement when you're still in the epidural space (and seeing epidural local, rather than spinal fluid, come out through your spinal needle). another issue is that once you create a 25g hole in the dura with an expanded epidural space because of your bolus(es) you'll get an unknown quantity of epidural local coming into the subarachnoid space due to pressure gradient. this in itself can lead to a high spinal. the other problem is that even if you inject a small dose spinal because the subarachnoid space is compressed by the epidural space, so your spinal dose spreads higher and quicker (again, pressure gradient from epidural space) so you're at higher risk of high spinal.
If you know all of this and are aware of it and plan for it, then go ahead. but those waters should be tread carefully. I personally am not a fan of doing a spinal after a bolused epidural, but I used to have partners do it routinely. personally I examine the epidural closely and if it hasn't worked well for labor, then I don't bolus it. I pull it and do spinal.
also, I've noticed personally that if I do a lower dose spinal after a bolused epidural (which by the way, I don't really do anymore for the reasons already stated), anecdotally speaking I believe the spinal doesn't last as long. this, in my opinion, is because the spinal space has been compressed by the epidural space, so less spinal med has spread faster both caudad and especially cephalad, but there isn't as much working on the levels that I want it working. it doesn't last nearly as long as a normal heavy bupi spinal. we better be in and out of the room in 45 min or so or I'm going to have to augment with IV meds pretty significantly.
also, since starting CSE my epidurals are better, less patchy, and I almost always am able to use the epidural for CS.