diff --git a/frappe/custom/doctype/custom_field/custom_field.py b/frappe/custom/doctype/custom_field/custom_field.py
index ee6e3b9c61..3126326636 100644
--- a/frappe/custom/doctype/custom_field/custom_field.py
+++ b/frappe/custom/doctype/custom_field/custom_field.py
@@ -40,6 +40,8 @@ class CustomField(Document):
 			frappe.throw(_("A field with the name '{}' already exists in doctype {}.").format(self.fieldname, self.dt))
 
 	def validate(self):
+		from frappe.custom.doctype.customize_form.customize_form import CustomizeForm
+
 		meta = frappe.get_meta(self.dt, cached=False)
 		fieldnames = [df.fieldname for df in meta.get("fields")]
 
@@ -49,7 +51,11 @@ class CustomField(Document):
 		if self.insert_after and self.insert_after in fieldnames:
 			self.idx = fieldnames.index(self.insert_after) + 1
 
-		self._old_fieldtype = self.db_get('fieldtype')
+		old_fieldtype = self.db_get('fieldtype')
+		is_fieldtype_changed = (not self.is_new()) and (old_fieldtype != self.fieldtype)
+
+		if is_fieldtype_changed and not CustomizeForm.allow_fieldtype_change(old_fieldtype, self.fieldtype):
+			frappe.throw(_("Fieldtype cannot be changed from {0} to {1}").format(old_fieldtype, self.fieldtype))
 
 		if not self.fieldname:
 			frappe.throw(_("Fieldname not set for Custom Field"))
diff --git a/frappe/custom/doctype/customize_form/customize_form.py b/frappe/custom/doctype/customize_form/customize_form.py
index c79c965aae..9f6996a660 100644
--- a/frappe/custom/doctype/customize_form/customize_form.py
+++ b/frappe/custom/doctype/customize_form/customize_form.py
@@ -401,22 +401,18 @@ class CustomizeForm(Document):
 		return property_value
 
 	def validate_fieldtype_change(self, df, old_value, new_value):
-		allowed = False
-		self.check_length_for_fieldtypes = []
-		for allowed_changes in ALLOWED_FIELDTYPE_CHANGE:
-			if (old_value in allowed_changes and new_value in allowed_changes):
-				allowed = True
-				old_value_length = cint(frappe.db.type_map.get(old_value)[1])
-				new_value_length = cint(frappe.db.type_map.get(new_value)[1])
-
-				# Ignore fieldtype check validation if new field type has unspecified maxlength
-				# Changes like DATA to TEXT, where new_value_lenth equals 0 will not be validated
-				if new_value_length and (old_value_length > new_value_length):
-					self.check_length_for_fieldtypes.append({'df': df, 'old_value': old_value})
-					self.validate_fieldtype_length()
-				else:
-					self.flags.update_db = True
-				break
+		allowed = self.allow_fieldtype_change(old_value, new_value)
+		if allowed:
+			old_value_length = cint(frappe.db.type_map.get(old_value)[1])
+			new_value_length = cint(frappe.db.type_map.get(new_value)[1])
+
+			# Ignore fieldtype check validation if new field type has unspecified maxlength
+			# Changes like DATA to TEXT, where new_value_lenth equals 0 will not be validated
+			if new_value_length and (old_value_length > new_value_length):
+				self.check_length_for_fieldtypes.append({'df': df, 'old_value': old_value})
+				self.validate_fieldtype_length()
+			else:
+				self.flags.update_db = True
 		if not allowed:
 			frappe.throw(_("Fieldtype cannot be changed from {0} to {1} in row {2}").format(old_value, new_value, df.idx))
 
@@ -458,6 +454,14 @@ class CustomizeForm(Document):
 		reset_customization(self.doc_type)
 		self.fetch_to_customize()
 
+	@classmethod
+	def allow_fieldtype_change(self, old_type: str, new_type: str) -> bool:
+		""" allow type change, if both old_type and new_type are in same field group.
+		field groups are defined in ALLOWED_FIELDTYPE_CHANGE variables.
+		"""
+		in_field_group = lambda group: (old_type in group) and (new_type in group)
+		return any(map(in_field_group, ALLOWED_FIELDTYPE_CHANGE))
+
 def reset_customization(doctype):
 	setters = frappe.get_all("Property Setter", filters={
 		'doc_type': doctype,