OSGi dynamics. Update of backing service.

[sinus]

Good day!
Could you please explain the following feature of OSGi dynamic
bundle support with spring-dm?
My configuration is described below (Felix 3.2.2):

g! lb
START LEVEL 1
   ID|State      |Level|Name
    0|Active     |    0|System Bundle (3.2.2)
    1|Active     |    1|AOP Alliance API (1.0.0)
    2|Active     |    1|JBoss Logging SPI (2.0.5.GA)
    3|Active     |    1|SLF4J API (1.5.6)
    4|Resolved   |    1|SLF4J Log4J Binding (1.5.6)
    5|Active     |    1|SLF4J Jakarta Commons Logging Over SLF4J Binding (1.5.6)
    6|Active     |    1|log4j.osgi (1.2.15.SNAPSHOT)
    7|Active     |    1|Apache Felix Bundle Repository (1.6.2)
    8|Active     |    1|Apache Felix Configuration Admin Service (1.2.4)
    9|Active     |    1|Apache Felix Gogo Command (0.8.0)
   10|Active     |    1|Apache Felix Gogo Runtime (0.8.0)
   11|Active     |    1|Apache Felix Gogo Shell (0.8.0)
   12|Active     |    1|Spring AOP (3.0.0.RC1)
   13|Active     |    1|Spring ASM (3.0.0.RC1)
   14|Active     |    1|Spring Beans (3.0.0.RC1)
   15|Active     |    1|Spring Context (3.0.0.RC1)
   16|Active     |    1|Spring Core (3.0.0.RC1)
   17|Active     |    1|Spring Expression Language (3.0.0.RC1)
   18|Active     |    1|Spring Test (3.0.0.RC1)
   19|Active     |    1|spring-osgi-annotation (2.0.0.M1)
   20|Active     |    1|spring-osgi-core (2.0.0.M1)
   21|Active     |    1|spring-osgi-extender (2.0.0.M1)
   22|Active     |    1|spring-osgi-io (2.0.0.M1)
   23|Active     |    1|pa (0.0.0)
   24|Active     |    1|pb (0.0.0)

where pa and pb are some custom bundles, with services and dependencies defined as:
pa

<osgi:service ref="s1" interface="pa.IServiceA"/>

pb

    <osgi:reference id="s1" interface="pa.IServiceA" timeout="3000" availability="optional"/>

After the bundle representing a back-end service (pa) has restarted,
this back-end service becomes available to the bundle pb

g! stop 23
g! start 23
g! inspect service requirement 24
pb (24) requires services:
----------------------------------
....
----
Registering bundle = pa [23]
Bundle-SymbolicName = pa
Bundle-Version = 0.0.0
objectClass = pa.IServiceA
org.springframework.osgi.bean.name = s1
osgi.service.blueprint.compname = s1
service.id = 25

2. However, updating pa makes back-end service unavailable to the pb bundle.

g! update 23
g! inspect service capability 23
pa (23) provides services:
----------------------------------
Bundle-SymbolicName = pa
Bundle-Version = 0.0.0
objectClass = pa.IServiceA
org.springframework.osgi.bean.name = s1
osgi.service.blueprint.compname = s1
service.id = 29
----
....
g! inspect service requirement 24
pb (24) requires services:
----------------------------------
...
<there is not such sevice>

Trying to call this service from the pb bundle results in

g! org.springframework.osgi.service.ServiceUnavailableException: service matching filter=[(objectClass=pa.IServiceA)] unavailable

Is such behaviour expected? And how can I attain truly transparent dynamic service reloading, retaining availability of services not only after bundle restarts but also after bundle updates?