the good old AP1310 is not anymore supported on WLC Release 7.4 and later (moreover is not possible to configure it in Flexconnect Mode) so the only chance is to replace it with the 1550 Series...I thought to use the model 1552I with Integrated Antenna but after checking the Datasheet of both families I've noticed that the antenna is gain of the 1552I is a lot weaker than the 1310:
Cisco Aironet 1310 Outdoor Access Point/Bridge with 13-dBi integrated antenna, ETSI config
Cisco Aironet 1552I Integrated Antenna Access Point has the following Integrated Dual Band Omnidirectional Antenna Radome (1552C/1552I) 2 dBi (2.4 GHz), 4 dBi (5 GHz)
So... I won't been able to reach the coverage of the installed 1310 with a 1552I and I have to go with a 1552E and its external antennas (i.e. External Single Band Antennas 2.4 GHz - AIR-ANT2413P2M-N= (13 dBi, dual polarized patch))
Has anyone already had experience with the 1552 after a migration from a 1310???
Did lots of 1552 deployments. You are correct in assuming you will need the external antennas as the built in antennas are likely not there for long reach, they are more suitable to create local coverage for clients (as opposed to be used for mesh infrastructure).
I have had deployments up to 2-3 miles. Nothing really long.
Check this link below. It is the distance calculator for 1552 using different antennas. Keep in minid that distance will depend on application throughput requirements, antennas, cables used, regulatory domain, terrain features etc.