There’s actually no justified reason for changing bearings if there’s nothing wrong with them. The fact that many owners simply knock out the old ones and fit replacements shows just how cheap new ones are. You can clean bearings in situ but it’s messy and frankly a bit awkward. Whether you opt to remove the bearings or not here’s the SP: if the balls and races are protected by a metal shield then this can normally be carefully prised out. Some plastic shields can also be winkled out but take extra care here. Wipe away as much of the old grease and dirt as possible with a lint free rag; then wash out the old grease with white spirit or similar. When all of the old grease has been evicted, wash the bearing with fresh clean solvent then remove this with brake cleaner and allow the bearing to dry. You can spin a dry bearing briefly and carefully but don’t overdo it. Notchiness, resistance or rumbling? Replace the bearing. Visible rust? Ditto. All being well then the bearing can replaced, with a ball race specific bearing grease. Cleaned and greased bearings can last for ages but are often easier and cheaper to replace.